That is a bug that can happen sometimes. If you're on PC, open the console and type "tcl" which will turn clipping off and allow you to "walk" back to the ground, where you can turn it off again.
If on a console, here are some tips form the wiki:
It is possible to escape this glitch by running towards the water after the conversation with Meridia. Another way to avoid fall damage is to position your character directly over the stone slab on which Meridia's shrine sits. because the pedestal is located underground some distance away from the shrine itself, the player will not be located directly above the shrine. If the player skips through the dialogue delivered by Meridia, they can walk around in mid-air until she finishes speaking, at which point she will drop the player again. For some reason, fall damage sustained from this "drop" is negated should the player land on the stone slab at the base of Meridia's shrine.