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Mars RoverMars Rover 

Developer console opening in lightning experince but when i switch to salesforce classic its not opening?

NagendraNagendra (Salesforce Developers) 
Hi Rover,

Sorry for this issue you are encountering.

The solution would be to reset the IDEWorkspace for the affected user. To do this, please:

1. Open the Developer Console as the affected user.
2. When the error appears, please the "Debug Script" button. If you don't have a "Debug Script" button you'll have to open the Browser's Developer Tools manually (please scroll down to the bottom of this Article for additional information).
3. Move to the 'Network" tab of the browser's Dev Tools.
4. You'll find a number of requests, if you don't, please refresh the browser. We're interested in a request with the following format (Please click on the link and copy the id):
5. The 'Id' value is the one we are looking for. Please copy that.
6. Open Workbench. Select "Utilities" and then "REST Explorer".
7. Do a GET request for "/services/data/v29.0/tooling/sobjects/IDEWorkspace/1deQ00000004HmJIAU" - remember to replace the "1deQ00000004HmJIAU" parameter with the "Id" relevant to your org that you obtained in step #5.
8. You will see the content of the affected user's IDEWorkspace.
9. Do a DELETE request for the same URL received on Step 7.
10. Re-open Developer Console
After following these steps the Menus should be available again and the error message should no longer appear.

For more information please refer to below link.
Hope this helps.

Kindly mark this as solved if the reply was helpful.