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.