Your limits could be messed up then. You need to relax the upper limit so it can shift into the largest cog. Way I do limits is shift into smallest cog and largest chainring. You can release the cable at this point since this is where you want the derailleur to be without any cable tension. Then shift into the largest cog and smallest chainring. You want to adjust it so the derailleur can shift into largest cog but not over. You want to make sure you can't shift beyond it and chain fall off. It's quite easy to do.

If the derailleur is bent so much that you can't shift up to the largest cog, then you probably need a new derailleur. Ebay has 'em cheap.

If it's derailleur hanger is bent, it's $20 to get it fixed at your LBS.