I downloaded .iso files and moved it to an USB (With K3D) and burned,in this way,i got a computer boot disk,but strangely my pc was not booted by it,my purpost is to crack a locked computer with it.Apparently it didn't work.

BIOS lock? BIOS boot order?
Please send some more informations.

The disk was set to the first boot device on the bios settings before you used it? anyway,that isn't the only way to crack pc.
Try it: http://www.freeisoburner.com/
Or with the help of this tutorial,that the pc can even be hacked.
https://www.winpwd.com/