For it to have worked for so long and then suddenly stopped, either there's a software glitch (most likely) possible from an app recently installed? Or a hardware failure (unlikely). I'm assuming you've tried rebooting the phone? If not, that's a good first step. Is the phone rooted? If so, have you flashed anything recently? Have you installed any apps that might have access to the radio/gps lately? If so, try uninstalling them and rebooting. If you're phone is stock (or rooted and you've eliminated any recent flash) and you can't find a guilty app, it might be time to bite the bullet and do a hard reset.
menu > settings > sd & phone storage > factory data reset
Before this, make sure you've sync'd successfully with the Google mother ship. Also, if you are rooted, check to make sure how to reset your particular rom. Some need to be reset via recovery.
If the issue persists after a factory reset, you're probably looking at some sort of hardware failure. FWIW, I like many other US based owners have had nothing but problems with Samsung's GPS implementation. This includes the Moment, Intercept, Epic and Epic 4g Touch. Poor/no signal, loss of signal and slow lock. The only major difference is that they didn't work right from the get go.
Good luck!