@agoltsman
Notice that Susan changed her answer to match mine. You need Form 3115, and probably a tax professional.
As far as starting it now and ignoring what was missed, you don't want to do that. The missed depreciation still lowers your "Basis" (similar to cost). So when you sell the property, you will be paying tax on the gain due to that depreciation, even if you did not actually take it. So you want to claim it.
