2001 Sierra power window

I bought truck used, it had aftermarket power windows installed from the original crank type, the other day while rolling my window up, about 3 inches from the top I hard something pop, and now the window will only move up and down about 2 inches. I took my door panel off, and physically raised the window shut, I am pretty mechanically inclined, but have never messed with power window before. Any suggestions, thanks!!!!