Ouch! That's a tough one. I sympathize with the rocky ground problem!
Can you get the base back out?
If you can, you could fill the hole with water, (It might soften what dirt IS there) then fill it up with loose dirt and re-screw it.
Unfortunately, if it's one of those BIG underground suckers...well, you might want to rethink it's location.
Sorry, I know that's probably not much help! :(
I can get the base back out alright. I did fill it with water. I didn't put any other dirt in there. I'm also having a hard time getting it straight. This is a major pain in the butt!