This should have an automatic reset. Turn the power switch off. Unplug the unit. Wait a couple of minutes. Plug it back in and turn on the power switch. If that don't work could be bigger problems.

The stylus (needle) is very sensitive to small motions and a larger motion from an errant touch can send a signal larger than usual to the amp. In turn, the amp will send a amplified signal to the speakers. Several things could happen, depending on what your gear has in terms of protection. Jackfish is correct in that the amp may have a protection circuit that shuts the amp down and needs a reset.

Another possibility is that the amp has speaker fuses that protect the speaker (the speaker may even have its own fuses. Those fuses may have blown. A third possibility is that there are power supply fuses internal to the amp that cuts the power to the amplifier, which may have blown. Check for all these.

Since you wrote that you will send the amp for service, the above may not be necessary for you to check, but you can check the next time (hope there will not be a next time).

I did something similar many years ago. I put the dustcover onto the turntable while it was playing (bad idea) with the amp turned up loudly. I accidentally hit the back of the tonearm and the stylus end went up and crashed down on the LP, sending a huge signal to the amp. Suddenly there a loud boom followed by a loud hollow pop sound, and then there was no sound. The amp was fine. Both speaker's fuses blew, protecting the speaker from damage.