I tried to take a sreenshoot of window 8 by two methods----------- 1. snipping tools--- it worked 2. By window shortcut------ win key + partsc------ but this second option is not working i think because when I press those keys togeather, no doubt the screen dims for 1-2 seconds but I didnt find the screenshoot on picture library--- screenshoot folders. Is that the picture goes somewhere else. can anybody please tell what is the actual problem.

It seems to be a keyboard shortcut issue or due to screen shot location settings issues. I would like to know some information about the issue, so that we could assist you further.
1. Have you made any changes to the computer prior to this issue?
2. Do you get any error code or error message?

I would suggest you to refer to the following Microsoft help article and check if it helps.

Take a screen capture (print your screen):

http://windows.microsoft.com/en-us/windows/take-screen-capture-print-screen#take-screen-capture-print-screen=windows-8

For further reference, you may refer to the following Microsoft Help article.

http://windows.microsoft.com/en-us/windows/use-snipping-tool-capture-screen-shots#1TC=windows-8