If the db is small you could post a copy for others to examine. Or not.
If in OS X can you could check the file permissions? Something may have broken there. RWX is good.
Does the db auto-login to a "data-entry" account?
"Electricity, the peril the wind sings to in the wires on a gray day." ~ Janet Frame
First I would re-install FileMaker Pro.
Then as Stephan writes, I would duplicate your original file and do a Recover on your duplicated file to see if have been corrupt.