Mixed Pairs

Hello, I'm replacing a standard outlet with a GFCI. The old one had a green ground, and 2 sets of black and white wires. I assume one set was load and the other line. But the outlet wasn't marked so I first took 1 set and connected it to the new outlet, but it didn't work (after restoring power). So I took that set off and put the other set on... still didn't work.. at some point of going back and forth I am afraid I mixed up the sets..
Using a volt meter one set has no reading, and the other 64.
If I cross sets I get 120 on one set and nothing on the other..

So again I assume that the 120 is my live side and the dead set is going upstream to another outlet, but the wiring in this building is not always standard (my 2nd problem is worse), so I wanted to make sure there was no other circumstances that would fit what I have described..

Thanks for your time

 

If you connect the dead set of wires to load side that part of the circuit will now be GFCI protected,you can wirenut the whites with a pigtail and blacks with a pigtail and put them on the line side,now just that receptacle will be protected.
Geo