Photograph stuck to glass in frame

You won't like my answer... there is no sure fire guaranteed to work way. The paper it was processed on and how long it has been stuck will factor in. Majority of attempts end up with torn or ruined photos.

What has happened is the photo got damp and the emulsion on the face of the photo paper has created a stronger bond with the glass than it has with the paper backing.

When it is recent damage freezing works if the back side of the paper is still wet enough to cause the water to expand back into the emulsion layer.

Because prints before inkjet printers were wet processed you might have some luck with soaking it in plain tepid water (but again, no promises made the print will survive)

The best way to prevent it from happening in the future is to always mount prints using a mat board between the print and the glass so the print doesn't touch the glass. And NEVER EVER spray glass cleaner directly on the glass. Spritz a clean cloth and wipe the glass with the very slightly dampened cloth. A drop or two of glass cleaner running under the frame will get wicked up by the paper of the photo.

eta: personally, I don't attempt to reclaim these images. Either I scan them, reframe using the original glass or a couple I've just placed the photo + glass in a bubble envelope to archive if I don't want to continue to have them on display.