It's been about 25 years since I've done any darkroom work, but back then I practically lived in one. I don't know how much the papers have changed over the years, but back then when one got stuck to the glass I'd just throw it in a tray of water, glass and all. The papers we used at that time (and presumably now) were pretty much impervious to water (they had to be to hold up to the processing) and the photograph would eventually float off the glass. It would tend to curl, but normally laying it on a towel face up would take the curl out.
If it's an irreplaceable photograph I'd ask someone who knows for certain though.