When you bought the second half, you should have started to depreciated the $45,000 (minus land) using your purchase date of that second half as the "placed in service" date.
As I said in my last comment, you'll need to figure out what you SHOULD have been depreciating and use that for your "prior depreciation" (even though you did not actually claim it). And due the mistakes, you may want to file an extension and go to a tax professional this year to review things.
