I could probably get you started.

If this helps...please mark as either best answer or like it please. This trigger is designed to send an email when a field is changed...You will just have to change the object and field names, I will highlight the important bits. It works by getting triggered when a field is changed and then uses an email handler to send the email.


Email handler class:

public class EmailHandler{


      //ClientPath Emails
      public static void sendTriggerEmail (List<String> contactids){
     
        Messaging.MassEmailMessage mail = new Messaging.MassEmailMessage();
        //set the email properties
        mail.setTargetObjectIds(contactids);
        mail.setSenderDisplayName('Your company customer service'); //change to whatever
        mail.setTemplateId('00X90000001F79Z'); //Id of the Email Template
     
        //send the email
        Messaging.sendEmail (new Messaging.MassEmailMessage[] { mail } );
        }
     
        
}

and the trigger:
 

trigger SendEmailtocontact on Service__c (after Update)
{
    List<String> lcontactEmails = new List<String>();
    Set<Id> sIds = new Set<Id>();
//Added
    Set<ID> accIds = new Set<ID>();
    for(Service__c qItr : Trigger.new)
{
        if(Trigger.isUpdate)
  {
            if(Trigger.oldmap.get(qItr.id).Last_Email_Sent__c!= qItr.Last_Email_Sent__c ) //basically says that if this field is changed it will trigger the trigger
   {
                sIds.add(qItr.id);
    //Added
    accIds.add(qItr.Account__c);
            }
        }
  //Will Never Execute as trigger will fire only if a record is updated
        else if(Trigger.isInsert)
  {
           
                sIds.add(qItr.id);
            
        }
    }
// Modified
    //for(Account accItr : [SELECT id,(SELECT id FROM Contacts) FROM Account WHERE id IN (SELECT Account__c FROM Service__c WHERE Id IN: sIds)])
for(Account accItr : [SELECT id,(SELECT id FROM Contacts) FROM Account WHERE id IN : accIds])
{
        for(Contact con : accItr.contacts)
  {
            if(!String.isBlank(con.id))
   {
                lcontactEmails.add(con.id);
            }
        }
       // No need to put this condition here.
  /*if(!lcontactEmails.isEmpty())
  {
   EmailHandler.sendEmail(lcontactEmails);
  }*/   
    }
//Put this here
if(!lcontactEmails.isEmpty())
{
for(Service__c serv : Trigger.new)
{        
        //Email handler Updated
        if(Serv.Last_Email_Sent__c == 'IPC Support Expiry') //Set critera if statement..Add others to set different emails based on different criteria
        {
        EmailHandler.sendTriggerEmail(lcontactEmails);
        }

}   
}
}