Maybe it decided to pull a major update when you weren't looking. You'll simply need to recover that password, somehow, using a desktop computer perhaps. Or else back up your data, perform a factory reset (as long as you're not running custom recovery) or...darnit there's probably a file out there somewhere that you can erase to let you create a new account but I can't remember what it was.
found it. http://androidforums.com/android-lounge/239687-help-delete-gmail-account-without-factory-reset.html#post2696390
Sent from my LG-VM701