Removed left rear wheel to repair leak. Removed grass from undercarriage. Replaced wheel. Tractor will not move under its own power. With engine off, I can freely push tractor even with brake on and in gear. Since the L100 is not an automatic, manual says it does not have a free wheeling switch. Did I hit a switch somewhere while cleaning? What else may be causing the inability to move when running and to be freewheeled when engine off and in gear with brake on?