Welcome to the Monestary, premsai!

"Everything was working fine and I am almost 99% sure that I have not changed something that would have broken the login function. I was in fact working on a form update function which updates the User table. "

Your best bet is to not guess at these things. If you use a Version Control System, check your commits and see what had changed, and if necessary roll commits back until you find out where things went awry.

If you don't use a VCS, I'd make a backup of what you have now, and restore the original software from a backup. Then I'd implement a VCS immediately to keep track of all code changes, as well as write a unit test suite so that changes can be tested reliably.