Why would I not have oil pressure after installing a high performance cam

I had a 585 high performance cam installed N my
C5 Corvette when it was done I had no oil pressure
and sounded like the lifters was making lots of
noise, the mechanic got back into it and said that
the lifter broke because somebody prior to me
owning it milled out the heads, Does this seem
correct information? I asked why only one broke
and got no answer, I asked if I should change all the
leftovers and he said no any ideas of what to do?
Thanks