#1
#2
Can we assume English is not your first language? Your post is hard to understand. Are you saying that your guitar string shorted out the amp because it touched the insides of the 1/4" effects loop or footswitch insert jacks?

Maybe the amp shorted out. Can you reboot or refresh the amp?
#3
You assume right. You see that "holes" above the "CUBE" signature (look at the link I've posted)? Part of my string accidentaly touched the board inside those holes, and it just went off. I cannot turn the amplifier on. Something just burned out.
#4
Unless you are good with soldering PCB's it will probably be cheaper to buy a new amp.

Check the fuses first and make sure that one (or more depending on some amps) are still good. You may have gotten lucky and only blew the fuse.