USB drives and external hard disk not recognized

My MacBook Pro had been detecting all pendrives/flash drives and my external hardisk without any problem till last week. Suddenly since yesterday I am not able to find them in the Finder. I checked the Preferences and as usual, the check box for Show External Disks is ticked. When I checked the disk utility, it shows the pendrive is connected but it is greyed out. If anyone has fixes, please help me by replying to this thread