In your AHU, you should have a transformer in there stepping voltage down to 24 volts. There may be a fuse and some have a little circuit breaker on them.
If the fuse is blown, you may have an issue that caused it to blow like a shorted relay coil, reversing valve coil, bad t-stat wiring, etc. or the transformer itself could be bad.
If the fuse is blown, you may have an issue that caused it to blow like a shorted relay coil, reversing valve coil, bad t-stat wiring, etc. or the transformer itself could be bad.