whats the error code? sometimes fb on android devices will log out and when u try to log in from your device, your facebook account wont let u. then you have to go on a cpu and log in there. youll be prompted that there was an attempt to log in from an unknown device. just follow instructions. this happened to my evo yesterday as well as my nexus a couple times when i had it. if thats not the issue, try rebooting it.