Touch the tubes - carefully. Is there any heat in the small tubes, the power tubes?
Swap the tubes left to right and vice versa. Is the same channel is still dead?
Unplug the amp, IMPORTANT!
Remove the bottom and see if there are any obvious wiring faults, Look at the speaker and input connectors. Do you any burn spots on the solder points? Charred resistor? caps(s)?
Single-ended class A Tube power amps are not very complicated pieces to repair.
They run very hot so there is a possibility a relatively inexpensive part might have failed from heat.
You may be able to fix it yourself.