If there is a lot of sand on a planet and it is incredibly hot for some reason and glass forms with two continental plates colliding into each other, it is possible for the plates to push up the glass in a similar way to how fold mountains are formed or simply, is it possible for a mountain to be made out of glass?
You might want to look up obsidian, volcanic glass, for some ideas of how it actually works in actual geological formations. There can certainly be big deposits of the stuff in a mountain - for example, Glass Mountain is named after the large deposits there, but there's always other stuff there to, to make up the rest of the structure.
Glass is brittle, it breaks and shatters before it bends and flows. If the glass was solid, you would end up with fractured chunks (scree or talus) before it could bend enough to be thrust upward to mountainhood. If it was molten, it would melt back down (obsidian flats) before it could cool. You can get a mountain that's mostly glass, if your conditions are just right or if there's interference - you just need a bit of something else to bind it together. Maybe even sand or sandstone, if that's what you've got the most of on hand.