there could be various reasons I suspect.
check your speaker connections , Check that there are no loose strands of speaker wire touching either the back panel of the receiver or the back of any connected speakers. Even one small strand of stray speaker wire is enough to cause the receiver to switch off, due to a short circuit.
there might be overheating issues...
Most electronics have a built-in fail-safe to protect against overheating. These fail-safe systems are designed to automatically switch the device off before the level of heat causes permanent damage to the circuits. Very often, the device won't be able to turn back on until the excess heat has sufficiently dissipated. Check if your receiver is overheating by placing your hand on the top and sides of the unit. If it feels uncomfortably or irregularly hot to the touch, then overheating is likely the cause. You can also check the front panel display of the receiver since some systems feature warning indicators.
if it doesn't work contact your nearest Marantz service center.
Hope this helps.