A sudden loss of compression in a four-stroke engine could signal a broken valve spring, trash getting inside the engine between the valve and the valve seat, or something worse. Yes, the trash can be in the engine compartment and suddenly get sucked into the intake midway through a flight. We've seen pieces of grass, seeds, and other types of trash stuck there. Occasionally, a valve spring breaks, and the valve stays open.

If the valve's not sealing, no compression.

You can most certainly send the engine to the shop to see what's gone wrong.