I was flashing my SD card with rasbian using etcher. while validating 5% my computer turned off due to power cut now when I turned it on the sd card capacity shows 42 mb so I formatted it nothing happened what to do? 😞
Etcher doesn't care what the "sd card capacity shows" (actually this is a function of your computer OS) or whether it is formatted or not.
It is more likely that the image has been successfully copied - the verification step is just an extra. 42MB is a normal boot partition. Try it in your Pi, although if you "formatted it" (how exactly) you have probably damaged the image.
Unless the SD Card has been damaged you can just run Etcher again.