If it's just the antenna broken, you could try switching them round into a different order, it will work with two but it's not easy to figure out which two!

Some of the vigor routers have a plug in wifi module inside. If the 2830n has that then maybe the module could have been dislodged?

A year later ?

Are you sure this is a single band model? Wrong firmware would stop the wifi, but then so would damage when
you drop it on the floor - you might have broken more than the aerial.

For what it's worth, I had a 2830n where the wifi stopped working exactly as you describe (Web interface works, but no signal). I fixed it by replacing the wireless card - see my other posts.

As the unit has fallen to the floor it may well be that the wireless card has become dislodged. This happened on one of my 2800 units when it was returned from Draytek after being sent back for a warranty reason (can't remember what now, but it wasn't wifi related!), however in my case, the Wireless sections of the menu didn't even show.