The good news is there are not a lot of areas where the gas can leak. With that said, use a soapy solution and swab all pressure connections and fittings to determine where the gas is escaping and repair. The beer in the keg probably degassed somewhat since there was no counter-pressure against it. Do not attempt to dispense any beer until the gas is replenished as this will degas the beer further.
Scott Zuhse, Instructor
Micro Matic Dispense Institute