I don't know that I know the answer, but I have noticed that occasionally mine will not pick up my wi-fi signal at home, even though I'm 10 feet from the router.
What I've found DOES work well, though, is WiFinder, available in the Market. If my Droid doesn't seem to be picking up my wi-fi, I'll run WiFinder and invariably it will then connect.
The other thing you can check is settings/wireless & networks. Poke around there to see if it's even seeing your wi-fi, and try connecting that way.
Not much, but hope it helps.
Edit: the wi-fi you're trying to connect to isn't password protected, is it?