Help!

THe shop just got my engine put back together - its had a rebuild done, with new internals, a machined and honed block, and resurfaced heads/valves, and the junk.

The engine cranked up fine, and at idle, the oil psi was at about 19psi. But there was a loud clacking coming from the engine - apparently it was the lifters clacking from the lack of oil. also, after about a minute of idling, the oil pressure drops to about 10psi

There was no oil at all on the top end of the engine.. what could cause this?

Thanks,