External hard drive not listed in My Computer. How to fix?

For some reason, my external hard drive is not listed in the My Computer section of Windows. It used to show up but now isn%26#039;t. I know it exists as I can drive to it through DOS and files can be retrieved from it. But when I try to change directory through the address toolbar to the letter it maps to, it refuses to find it still. Anyone know how to fix this?|||I have things not show up like that when the computer needs restarted. Running low on memory or something.|||Connected thru USB?