I can't find my external hard drive

Hello,
My external hard drive used to show up on my explorer list and also in my computer connections. Now I can't find it in either one. I tried connecting to a different USB port but that hasn't helped. Why suddenly did I lose it and how can I find it again? I use Windows Vista but I am sure that is not the problem, because I had it prior to losing sight of my external hard drive. I am sure when I purchased it, it was plug and play. ie. nothing to download to make it work.