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.
A heat sink is just a finned heat-conducting device for radiating heat to the air which should be flowing freely through the unit's ventilation holes. It can't break in normal use. But it CAN get clogged with dust reducing its effectiveness.
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"Choose a location that is well ventilated (with at least several inches to both sides and behind), and that will provide a clear line of sight, within 25 feet/8 meters, between the receiver's front panel and your primary listening/viewing position-this will ensure reliable infrared remote control communications. The T 743 generates a modest amount of heat, but nothing that should trouble adjacent components. It is perfectly possible to stack the T 743 atop other components, but the reverse usually should be avoided. Nonetheless, it is generally preferable that the T 743 stand alone. It is especially important that sufficient ventilation be provided; if you are contemplating locating the T 743 within a cabinet or other furniture, consult your NAD audio/video specialist for advice on providing adequate airflow."
If you're lucky, you haven't finally toasted it. If you have, a hands on tech will need to sniff sround (literally) to find the dead component(s).