I run into that problem regularly, you have water frozen in the line in a low spot between the air compressor and the air dryer. I disconnect air line at compressor pour in some airline antifreeze alcohol down air line, and then crack loose the air line at the air dryer where the line comes in from air compressor, then I will also end up heating the line and fitting at the air dryer soon the alcohol will start leaking out the loosened line. Start engine you will now hear the compressor air coming out the line, tighten the lines up and now air pressure will build.