Possible reasons this might happen include:
Both the compressor bypass being open and a flipped circuit breaker can be seen remotely by looking at the pressure gradient across the compressor while its running. No gradient means no compressor. If you are on site, both of these are easily fixed.
A clog in the pre-cool lines can be diagnosed by warming up, and collecting the mixture. If you see more than the expected pressure in the dumps, there might be a leak. Scrub the mixture (see [sec:maint.scrub.full]§sec:maint.scrub.full).