what is my transmission shot or an electrical problem?

im thinking a shift solenoid may of came unplugged or went bad we bought a rebuilt transmission and one of the solenoids either came unplugged or didn't get plugged in in the first place,, wouldn't shift into overdrive,, dropped the transmission pan an found a solenoid that wasn't plugged in,, worked like a charm after that,, if you can limp it to an advance or get your hands on a scanner id scan it, and see what you find, also that Tom posted about the fluid is a possibility as well, but if its low im questioning why its low since it should not be losing fluid and they should of made sure it was at the correct level before they released the car to you and making sure the fluid is at the correct level is thee easiest part of rebuilding a transmission