PARK AVENUE ULTRA WINDOW Won't Go Down

Today as I was lowering the driver side window, I heard a loud pop and only the right side of the window will start to go down. I was able to close the window and tried several times with the same result; the left side of the window will not go down. The window motor seems to be working OK but something is keeping the left side of the window from moving. The loud pop suggests to me that something broke and is in the way of the left side of the window.Any thoughts on what this might be?
Thank you.

It is likely that one of the "hot dog" guides broke in the lower channel or broke off the at the glass. The interior door panel must be removed for inspection. In the interim, do not attempt to move the window again or glass breakage may occur.

Like Blue says, you won't know until you remove the door panel, and the water barrier, then operate the window up/down.......could be a guide like Blue says, or could be a bad regulator........