The rear window slipped down and won't go up. Could it be the regulator?
Yes, it sounds exactly like the window regulator. The window is held into the door by the regulator which moves up and down by the window motor. This motor is controlled by the window switch or it is manually cranked up or down. Some regulators have pulleys and steel cord used to move the window up and down and this steel line can get wrapped up or stuck in the pulleys, not allowing movement. Also, the regulator could break all together, no longer supporting the window which allows it to move freely. A proper windows inspection by a certified mechanic will allow you to find and fix this issue.