Make sure that you enter the same account name as you did originally. FileMaker enters your user name as the default, so if you switched computers, you may find that your system is automatically entering a different user name as the default account name.
Make sure that your password is being entered with the same capitalization as you did when you changed your password. Passwords are case sensitive.
One more case study in why you should make frequent back up copies. If you made a back up copy just before you made this change, you could always toss the copy you can't open and revert to the back up.