Unit stopped working condenser & air handler -- breakers are OK .hmmessage P{margin:0px;padding:0px}body.hmmessage{font-size: 10pt;font-family:Verdana} RHSA HM3617JA 3 ton Vertical mount

Must be a transformer gone out.
TRANSFORMER AND THERMOSTAT


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Your question is very vauge. If you are "absolutely" sure breakers "in all locations" are fine, then I will give you the troubleshooters way to find problem. BE AWARE, SOME OF THESE SUGGESTIONS REQUIRE KNOWELEDGE OF ELECTRICAL CIRCUITRY. UNQUALIFIED PERSONS SHOULD NOT ATTEMPT.
Before getting technical, if you have a digital thermostat, it may be that you have one that requires batteries. replace these.

1. if nothing runs and breakers (high voltage) are ok, then you most likely do not have 24V control voltage or 24V switching. This leaves us to look at transformer in air handler or thermostat problem. The transformer in your air handler is usually where all your control power come from unless you have a battery powered Thermostat.
2. if you hear a "very faint" hum in your air handler or furnace, it is most likely your 24V transformer is OK. If you do not hear a hum, there may be a power problem. This could be bad transformer, bad fuse for transformer, or no power to transformer, or simply a good quiet properly operating transformer. An electrical meter is really only way to tell.

Any further checking would require a professional.