Bad Hadraulic Lifters, Maybe?

Ok, so the engine ran just fine. Then you pulled the pan and pump out to replace it with a new one. Now you have Zero oil pressure, engine is knocking and running bad. Im sorry to say, the engine is SCREWED. The oil pump is either broken or not installed right? Did you have to remove any main bolts to install the tray? Did you forget to torque them back down if you did remove them? Did you fill it back up with oil?

Its not magic, you messed something up and you need to pull the pan off and check out what you did wrong.