If the protection light is on, this usually means there is a genuine fault and it's more than 'just a fuse'.
The protection would be triggered under a number of conditions, but if it's coming on at power on, then in all probability, there is a fault causing DC voltage to appear on the output terminals (but because the protection is on, that DC voltage is not appearing at the speaker terminals thankfully!).
You could try disconnecting all speakers, ensure that the volume is turned right down, and then power on.
If the amp remains in protection, that eliminates any speaker / cable faults. If the protection light goes off, then power it off and reconnect one speaker at a time, powering it on at each stage, until it goes back into protection.
Be sure to remain close to the power button in case of any loud (usually humming in the event of a fault) noises being emitted from your speakers, so you can power it off promptly.
As for fuses - unless you were pushing the amp exceptionally hard, or a short developed in a speaker cable at high volume, it's unlikely (but not impossible) the cause will simply be a blown fuse.
WARNING: The are some very, very high voltages present in these amplifiers, particularly the higher output versions such as the NAD 272 / 218 power amps and even the C35x series. These voltages can be present for several minutes after powering off (in some cases several hours) due to the large power supply capacitors employed in these designs. Do NOT remove the covers unless you're absolutely certain you know what you are doing. The manufacturers warning labels are there for a reason!
Hope this helps.
Tony.