It is possible that it does not have gas, that the compressor, this one has problems or has some plugging, see if the freezer inside fan rotates, also can remove the inner cap of the freezer (where the fan is) and see if the frost evaporator, see if they heat the sides or back of the equipment, etc., comment result, for the other tests specific equipment is required
You can also put a service valve to prick, these valves are screwed and placed in the service appendix, then put pressure gauges