Thanks Greg for helping.

I really don't understand what I am missing, can you please look into this.

// currently using this class to test. I have a VF related to this which shows table of Key records whose details to be sent


public class RenewalEmailSender_UsingTemplate {

    public List<Key__c> lstKeys {get; set;}
    private Map<String,Key__c> mapKeysUnique;
    private String[] toAddresses;
        
    Transient Messaging.SingleEmailMessage mail;    
    Transient Messaging.EmailFileAttachment efa;
    Transient Messaging.SendEmailResult [] r;
    Transient List<Messaging.SingleEmailMessage> sem;
    Transient Set<ID> targetIdsFailed;
    
    public Boolean test;
    
    public RenewalEmailSender_UsingTemplate(ApexPages.StandardController controller) {
    }
    public RenewalEmailSender_UsingTemplate() {
    }
        
    public void initialize() {
        test=false;
        mapKeysUnique = new Map<String,Key__c> ();
        lstKeys = [select Name,Reseller__c,Maintenance_Renewal_Date__c,OwnerId,Reseller__r.Name,End_User__r.Name,
                          Maintenance_Renewal_reminder_sent_date__c,Email_ID_of_Reseller__c,Owner.Name,
                          Opportunity__r.Account.Name,SKU__c,Product__c,Quantity__c,Reseller_Contact_Id__c,
                          Product_Key_V3__c,Product_Key_V4__c,Status__c,Maintenance_Renewal_Price__c 
                          FROM Key__c WHERE
                          Maintenance_Renewal_Date__c > NEXT_90_DAYS AND Maintenance_Renewal_Date__c <= NEXT_N_DAYS:120
                          ORDER BY Reseller__c];
        for(Key__c k : lstKeys) {
            if(k.Email_ID_of_Reseller__c != null)
                mapKeysUnique.put(k.Email_ID_of_Reseller__c, k);
        }
    }
        
    public void sendMail() { // this method is called on button click
        sem = new List<Messaging.SingleEmailMessage> ();
        if(! mapKeysUnique.values().isEmpty()) {
            for(Key__c k : mapKeysUnique.values()) {
                sendAndAttach(k);
            }

            sendInBulk();
            update lstKeys;
        }
    }
    
    private void sendAndAttach(Key__c k) {
        System.debug('### (RenewalEmailSender) k: ' + k);
        mail = new Messaging.SingleEmailMessage();
        
        if(k.Email_ID_of_Reseller__c == null) {
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.FATAL,'Error while sending email - No TO address specified ('+
                                 k.Name));
            return;
        }    
       
        mail.setSubject('Maintenance Renewal for next 90 days (' + System.Today() + ')');
        mail.setTargetObjectId(k.Reseller_Contact_Id__c);
        mail.setTemplateId('00XR0000000IH56');
        sem.add(mail);
        for(Key__c k1: lstKeys) {
            if(k.Reseller__c == k1.Reseller__c)
                k1.Maintenance_Renewal_reminder_sent_date__c = System.Today();
        }
    }
    
    private void sendInBulk() {
        r = Messaging.sendEmail(sem,false);
    }
}

The VF email template

<messaging:emailTemplate subject="90 Day Maintenance Renewal Notice (Reseller)" recipientType="Contact" relatedToType="Contact">
    <messaging:plainTextEmailBody >
        NOW IS THE TIME FOR YOUR CUSTOMER'S PRODUCT MAINTENANCE RENEWAL...............
    </messaging:plainTextEmailBody>
    <messaging:htmlEmailBody >
        <h2><b>Now is the time for your customer's products maintenance renewal.............</b></h2>
    </messaging:htmlEmailBody>
    <messaging:attachment filename="Maintenance Renewal Notice - {!relatedTo.Name}" renderAs="PDF">
        <c:MaintenanceRenewalComponent contId="{!relatedTo.Id}" />
    </messaging:attachment>
</messaging:emailTemplate>

 Th VF component:

<apex:component controller="MaintenanceKeyController" access="global">
    <apex:attribute name="contId" description="The ID of the Reseller for whome we want to send email" assignTo="{!contId}" type="Id" required="true" access="global" />
    <table border="1" cellspacing="0">
        <tr>
            <th style="background: #BABABA;"> End User </th>
            <th style="background: #BABABA;"> Maintenance Renewal Date </th>
            <th style="background: #BABABA;"> Product </th>
        </tr>
        <apex:repeat value="{!lstKey}" var="key">
            <tr>
                <!-- Some text here -->
                <td> 
                    {!key.End_User__r.Name} 
                </td>
                <td>
                    <apex:outputField value="{!key.Maintenance_Renewal_Date__c}"/>
                </td>
                <td>
                    <apex:outputField value="{!key.Product__c}"/>
                </td>
            </tr>
        </apex:repeat>
    </table>
</apex:component>

 And it's controller:

public class MaintenanceKeyController {
    public List<Key__c> lstKey;
    public ID contId {get;set;}
    
    public List<Key__c> getlstKey() {
        if(lstKey==null)
            lstKey = [select End_User__r.Name,Maintenance_Renewal_Date__c,Product__c from Key__c where Reseller_Contact_Id__c =: contId AND
                 Maintenance_Renewal_Date__c > NEXT_90_DAYS AND Maintenance_Renewal_Date__c <= NEXT_N_DAYS:120
                 AND Maintenance_Renewal_reminder_sent_date__c = null];
        System.debug('-------- contId: ' + contId);
        System.debug('-------- lstKey: ' + lstKey);        
        return lstKey;
    }
    }
}

I checked it over and over, but couldn't find the mistake. The email is not received that is the only thing I could find.

Any help will be greatly appreciated, as this is very important for my implementation.