Mistake on tax return already filed - please help!

I have a mistake on tax return already filed! I think I've overpaid and I'm not sure what to do. I did not take into account all the tax deductions and now I find myself out of pocket. Is there a way to recall my filed tax bill so I can fix the error and somehow get my money back? Whats the process to fix a mistake on a tax return which has already been filed? Help!