I replaced some old wiring. I have a main line and a line for my ceiling light coming into my junction box. I connected the black wires together and the whites together. I had no power until I connected the copper grounds together. What do I need to change?

Please provide us with more details. Is there a wall switch in the circuit?

It sounds as if your neutral and ground are cross connected.

If you took your reading to ground then you wouldn't have a connection to ground until you made it. Read between the black wires and the white wires should = 120 VAC or close to that number. If you don't read 120 VAC between those two, you have something either wrong in the upstream connections or you have spliced into a switch leg and I doubt either is the case, but if it is the case then it appears someone didn't know what they were doing in a previous connection. More information would be helpful.