870 won't fire/action won't lock

So I'm in a bit of a pickle. My beloved Remington 870 Police Magnum has been in a pile of parts for almost a year now, waiting on the right stock set to be installed. I had some craptacular "ATI" stocks on it, and I hate them. For my birthday last week I got a set of Blackhawk! NRS stocks for it. Other than some fitment issues with the pump furniture, it went on ok. However, now I can't get the gun to function.

By "pile of parts" I mean I had the bolt, bolt carrier, tube assembly, and the stocks and barrel off. I didn't delve any deeper into the receiver. However, now I cannot get the action to "lock" at all, with a shell in it or not. The slide release button is stuck "in" and won't release either. Also, the hammer won't trip. Not sure what I've done, but something isn't right. It worked before the dissassembly, other than every now and again the action would catch and you'd have to wrestle it to open the receiver. Not sure if this is related.