If the print is from the era when photographs were processed with wet chemicals, then it should stand up to being re-soaked. Taka a tray (larger than the print) and fill with cold to tepid water. Lay the print flat and keep submerged for 15 mins or so. If the paper doesn't come off on its own, tease it with an artist's soft paintbrush.
Caution 1: If the stuck paper is dyed a colour, it could come out into the print.
Caution 2: If the print is very old or has been exposed to strong UV light, the gelatin layer or the paper base may have degraded and could disintegrate when re-wetted.
Good luck.