Step 1 - turn on amp but plug nothing into the input. If it does not buzz it is not the amp. Expect a litle buzz from an unloaded input like that.

If it did buzz - do as PDQ said.

The other very possible source of such a buzz - in the amp but not expensive or troublesome to fix (usually) may be the input socket. If the earth wire from that inside the amp that goes to the preamp section has come off a loud buzz is likely. And even a short of the input will not silence it.

Turn the amp off and unplug it and wait for 15 minutes or so for all capacitors to discharge. Have a look for any loose wires. Do not reconnect unless you know what to reconnect to! With a good digital meter measure resistance from the earth side of the input socket to the chassis. If that is not zero, you have a wire off between the socket and the chassis.

If it is a REALLY REALLY LOUD Buzz, suspect something to do with the power supply, possibly a rectifier or a capacitor. If you do not know what you are doing, do not play with it, you might kill yourself.

If PDQ's manoeuvre did silence it, try (a) a different guitar lead and (b) a different guitar.