First.......relax, dont worry, have a homebrew (or just another beer) and leave it alone.
I'd guess you used a dry yeast which might have fermented fast. No air lock activity is fine. If the water level is high enough to prevent air from passing in/out without moving the air lock than you're fine. I'd personally focus more on the temperature than anything. As long as it's 70 or lower just leave it alone for the next 3 weeks.
oh, and I'd suggest planning your NEXT brew during this time. Your first one is great but when you have nothing to follow it up you're dry for another month.