If memory serves, one of the colored banners in the entry hall of Scourg Barrow can be used, and it'll apply a temporary levitation effect to your character. Lots of dungeons will have similar objects such as skulls or chains with levitation effects near areas where the player will need it.
The first time I started the main quest and got to this quest, I mistakenly jumped in the wrong coffin and landed in that big room with two small connecting rooms, but no exit. I overwrote my save game after killing a mummy there.
If I recall, there's an object you can click to get levitation to go back up.
I never thought of it and after searching for a long time for a secret door, I scrapped my character and started a new one.
(I didn't know cheats were possible at the time. Didn't have Internet either to find how to get out.)
That frustrating event made me quit the main quest and I sticked to the faction quests. Until two years ago when I thought that "maybe I should finish this game at least once since I bought it. At least once."