Since you have told us only the minimum about what you did, I have to make assumptions. Presumably, (1) it is locked in one gear and (2) you removed the shifter to remove the transmission and engine. If so, my first guess would be the shifter. Something got boogered (sorry about the high-level technical terminology) on reinstallation of the shifter. I have a hard time thinking of any other cause, but of course my level of imagination is only so good.