Why did my amp just crash and burn ?

if yo need servicing I'm in the New Jersey NYC area.

itsjustme is correct turn it off. I’m still guilty of changing out interconnects as long as I change the source.  Most of my amps are tube and I don’t want to wait to turn it back on. I’ve learned to turn amp off when playing with speaker wires I double, triple even quadruple check speaker wire before turning amp back on. I had to learn the hard way when I blew up the McCormack. 

You may have grounded positive speaker wire to chassis while handling speaker cables. Like someone said open up the amp look for more fuses. Look for burnt resistor, black marks on the board, look at power supply electrolytic caps usually the tops where the cross is are swollen, even swollen film caps. Look at it as a learning experience what not to do, that’s what I had to do, Good luck.

Nice to see you all trying to help.

If the amp was on and the speaker cables not tight you created an ever load of resistance to the circuit ,check the fuses first ,sometimes you get lucky see outside fuses, and inside with a volt meter  pull each one out while unit s Unplugged .

We all make mistakes ,Always make sure all cables are secured good before turning the unit on Even swapping a digital usb cable I turn everything off  first.

It doesnot take much to cause a spike or surge , that is most likely  what happene. 

Well, was the main fuse blown.

If not , then there is a other issue. Do not attempt to fix it yourself.

Just getting the chassis open is difficult, and there's nothing you can repair yourself unless you are experienced in such things.