I have an article here for you that describes this issue on your particular model of computer and can provide troubleshooting steps:
This article might also provide some helpful steps:
Take care, and thanks for visiting the Apple Support Communities.
Thank you for your response.....actually the problem that I was facing vanished into the thin air just the way it appeared....nevertheless I am going to run the software update & go through the trobleshoot procedure so that next time I know what to do if this problem arises again.
Thanks & Warm Regards,