Internal error is coming after publishing application suddenly even after undoing changes. I noticed that I have not done anything that caused this issue, it suddenly happens and after undo the last time it worked properly

Hello Leslie,

Internal error page happens when you have an exception that is not being caught at the local where it happens, ending in the global exception handler.

You can look into the logs in service center to see more details about the error.

If it is related to the session, logout and login may help. 

Other than that, is debugging you will find the answer.

Without more information, is difficult to help.