after you check to see if the derailleur is bent, try checking your cable slack and your limitation screws. although it is unlikly that a crash could have changed this setting, stranger things have happened. if the derailleur is bent, go buy a new one unless you think you can just bend it back and you are ok with that. if your derailleur hanger is bent, you may need to take it to a shop to rebenf it if you are not comfortable doing it yourself.
adjustment, then if not, replacement. without pictures, my help can only be general.