0 ohms is a dead short. That's bad unless you're measuring a single conductor end-to-end.
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Did it ever work?
If not, EVERYTHING is on the table as a possible cause. This is a big dumb amp with nothing but a power switch. Perhaps your source is the issue. Or the stereo/mono switch.
If so, what preceded the failure? Did a control get touched? Smoke?
The Hafler could be protecting itself.
Generally speaking, an amp attempts to protect itself from heat, shorts, overloads and operator exuberance by refusing to turn on or stay on.
Overloads can be from excessive periods of high output or marginally low impedance loading by the speakers; and shorts would be wiring issues or a speaker blowing up.
You should be able to feel if it's hot. WHY is it overheating? Make sure it has sufficient ventilation on all sides and that vent holes are not blocked by dust balls. Ensure the fan (if equipped) is running as designed (some only operate on demand). Clean dust and debris from it.
If the amp comes back on after cooling, you're lucky. They only have so many self-protection cycles in their lives so continuously resetting or cycling their power without addressing the cause can do more harm than good.
If it protects immediately on a cool power up you should disconnect the speaker connections and try it 'naked'. If it comes up then diagnose which lead(s) are shorted. If it does not come up the problem is internal and should be left to an experienced and competent hands-on tech.
Check for loose speaker connections at the speaker as a root cause for intermittent shutdown.
NEVER USE MAXIMUM VOLUME FOR TROUBLESHOOTING.
Turn the SOURCE volume control to something medium.
The errant switch, control or condition may change and you will suddenly release the amp's full power and possibly destroying your speakers. If a signal isn't audible at 1/2 volume it's probably not there.
Carefully examine the front panel for clues like a misplaced Mute or Tape Monitor control or Multichannel Analog Input selected.
There is a good chance that a common control may have developed a high-resistance or 'dead' spot through idleness and is causing your symptom. Turn the POWER OFF and operate every control throughout its range a number of times, especially rarely-used ones like Tape Monitors and the Mute control.
Turn the volume to something reasonable and see if that helped.