It might - it is promising. A well designed app would either silently remove the ; or would return a proper "bad input" error message. That it blew up (500) is a good sign.
It doesn't necessarily mean that is vulnerable, however - they could be returning 500s in case of bad input, for instance. But it certainly suggests you may want to try some more. Try putting an '19 or sleep(2) = 1' (no quotes) - if it takes 2 seconds longer to respond, you are in like Flynn.
NOTE : this should be on either a site you have been asked to test, one you control or a demo site designed for cracking. Doing this on some else's site could result in charges.