1. Your preferences have become corrupted. Press and hold Control - Shift - Alt immediately upon cold-starting Photoshop, and answer the prompt to delete current prefereces. If you don't see the prompt you haven't gotten the keys pressed quickly enough.
2. You've lost/deleted/renamed/moved/fooled with files in your Photoshop installation area. Restore it from a backup.
3. It doesn't sound like an activation issue, but maybe... Assuming you meant Photoshop CS2 when you typed "PSC2", you could install a fresh copy of this: