My Seagate external hard drive wont show up on my computer. What should i do?

It has worked for a long time now and i had it plugged in and it just disappeared off my computer. The light is still on showing that it is connected i just cant access it. The same thing has happened to my Western Digital My Book terrabyte external hard drive. When i go into device manager it says usb to ata/atapi bridge or something for the Western Digital and ST9320325AS ATA device for the Seagate so it recognizes somethings there but wont let me access anything and i already have a ton of stuff on both so i dont want to format it unless theres a way to back it up and get the stuff later. Does anyone know what to do?