Last year I bought a used ST 3000. It had good compression; started and ran fine on the bench. I put after run oil in it and stored it inside the house for nine months. Now, it has very weak compression. Is it likely that a stuck ring is the problem?

If the engine has been idle for a long time, then it's possible that the ring has become stuck. You'll need to disassemble the engine and clean it out. Fuel residue can gel and become like glue, keeping the ring from properly expanding. If the engine turns over OK, you can just use an electric starter to get it going. Once it's running, the fuel flowing through the engine, as well as the heat of combustion should relieve any sticking.