Depending on what sort of groupset you've got (that's techspeak for what sort of sprocket it is) it will probably be easier to replace the sprocket (around �8) if that's not feasible you could replace the whole assembly, that may not be practical and the bike might be scrap. Take it to a bike shop and ask their advice. Advice is usually free! Take it from there. It's not always practical to straighten sprockets and if you do you'll get thro chains like they're going out of fashion.........