No Way Out?
I'm afraid you've fallen straight into the Oubliette.  Good thing there wasn't a sharp stake awaiting you at the bottom.  Perhaps if you retraced your steps ... yes, c'mon, you can do it ... jump really really high, up to the ceiling.  That's it ... quickly now, before a trapdoor falls into place and you're locked in here for all time to come ....
oubliette complimens of webshots.com  --  concept and layout copyright Valkyrie Grant