Finally, this problem get solved with removal of temparory fies from Windows --> temp folder. Whenever I get the problem, I remove this and it solves my problm.
IT also started giving problems in no. keys, getting symbols only. also was not able press . key. I pressed alt+fn+windows key and that solved my problem - got this solution from some website. I would like to understand what happens when all the three keys are pressed together - as I tried this as it was specified in the website.
Finally, this problem get solved with removal of temparory fies from Windows --> temp folder. Whenever I get the problem, I remove this and it solves my problm.