Hopefully it was a momentary spike that stuck it to one side. Often you can un-stick it by giving it a good rap. If not, then disconnect the battery, remove the guage from the dash, and then the glass and trim ring can be removed from the guage by giving it about a quarter turn.
This should give you access enough to un-stick the needle mechanism.
If you hook it back up, and it does it again, then you've got problems. I.E. bad voltage regulator, or shorted cell in the battery.