Look through the tube with a fiberoptic scope to see if it has changed positions. Most likely it has migrated deeper, especially if you are on a face pillow on an OSI bed or Jackson frame. If you are on a horseshoe or in Mayfield pins, you should be able to get under the table and trouble shoot. In that scenario, usually the weight of the circuit is pulling the tube out so always secure it to the frame in some way.
If it hasn't, you have to look at the possibility that it isn't the tube. Always make sure that your patient has some sort of bite block before turning prone so that positioning, repositioning, edema, etc. do not cause the tube to kink.