I pluplugg atx power supply to 220v outlet with the voltage selselectiswitch ono110v. I make bang sound and room switch dropped.. How can I repair it?

 

Welcome to AAC!
You might be lucky and find just a fuse has blown. Unfortunately, unless you have some electronics experience, failure of any other component(s) may mean that repair would not be practical or safe. If you do take the covers off, make sure to remove all power first. There are LETHAL voltages in an ATX supply. Capacitors can retain a high voltage even some time after power is removed.

 

Probably buy a new one. They are quite cheap now.
But at least, fuse, bridge rectifier, FETs, maybe controller IC and misc minor parts.
And the capacitors may be shot too. Unless you are experianced with electronics, buy a new one. These switch mode power supplies are quite dangerous to work on.

 

There's probably a MOV across the mains input which, if present, will be blown and would blow the fuse, but it might possibly have protected everything else.

 

Unless the fuse has saved the psu, your chances of repairing it are almost Zero, it will have taken out the two capacitors, and more than likely the two mosfets..