Amp troubleshooting tips
The things most likely to fail suddenly in a tube amp are fuses, tubes and jacks - roughly in that order. Keep spares for the first two and try substitution before dragging your amp to a tech.
Failed fuses are pretty obvious - if the amp doesn't light up when you turn it on, check the fuse. (And, of course, the power. Don't laugh. I worked as a repair tech when I was in high school in the late 60s, and I actually did get a few service calls where the set had been unplugged.) Blown fuses usually, but not alway, look burnt on the inside of the glass. When in doubt, it only takes a few seconds to try a spare. If the fuse blows again, don't keep popping spares - see a tech.
Failed tubes are a bit harder to isolate. If a rectifier tube (not all amps use one, BTW) or power tube is dark while its neighbors light up, there's a problem. Preamp tubes may not glow visibly, but you can check them for warmth after they've been on for a few minutes. Any kind of milky coating on the inside of the glass is an indication that the tube has lost its vacuum. This is how tubes usually fail if they fail suddenly - shock and vibration mix with stress in the glass or the glass/metal seal to cause this problem. If you're getting any kind of sound at all out of the amp, then the power stage is alive and one of the preamp tubes has probably gone bad.
Jacks can fail mechanically, especially the jacks used for FX loops. If your amp suddenly stops making sounds without smoke, fireworks, or a blown fuse, try running a guitar cord from the FX send to the FX return jack.
Sometimes swapping a preamp tube can cause your amp to hum where it didn't before. 12AX7s can be found (depending upon manufacturer) in both "folded" and "spiral" heater versions. The former can induce more hum when used in the the first gain stage.