moon roof problem

My wife drives her 1990 Galant GSX with great pride. She should, the car is in abolutely perfect condition except for a problem with the sliding roof window which jammed about a month ago. I have been reluctant to force the window open, concerned that I might not be able to find the necessary parts to repair it and not be able to close it again. We have experienced constant rain here this summer and it would not be welcomed by her to drive with an umbrella sticking up through the top.
Are there any similar experiences and what was the cause?