I bought an external CD buner from Amazon.com. It's a Memorex 52/48/52 burner. It was working well for last 15 days but since a couple of days back I cannot see the Drive on my computer. If I unplug all the cables from my PC and replug again, it will show again. What has caused this to happen? How can I fix this problem?

I assume it is a usb external burner? Try connecting it to each of the usb ports. Try the back ones first.

If you have a usb keyboard and mouse put them together on 1 set of usbs (usually a set of 2 stacked). Put the external burner on the other usb stack, and vice-versa - to check if there is a difference.

If you have a usb to PS/2 adapter use that for the mouse and keyboard, to free up as many usb ports as possible.