How to restore log in?

After having to shut it down by button for freezing, my MacBook won't let me log in. It just doesn't recognise my log in ID and password. It let's me log in as a guest but every time I try to change security details it asks for my administrator details and again it doesn't recognise me. It has been asking me to make space cause the start up disk is almost full and I was just about to back up my files to delete them from the machine when it froze and won't force quit.