My Seagate external hardrive is not responding. It is not showing up in my computer nor in device mamagement. I have tried it in several different computers, in different usb slots but nothing comes up. I have read through all the advise here and tried it, but nothing seems to work. I do not want to loose all my files on there are sentimental pictures and thousands of music files on there.
Unfortunately you have done what so many people do - or rather don't do. You have not kept your precious files backed up on a second drive. Never, ever save your data on just one hard drive because it can fail at any time.
With any luck, the only part that has failed is the SATA/USB Bridge Chip inside the enclosure (it's a small PCB). If you can extract the hard drive from the enclosure and remove the bridge chip, you may be able to fit it inside a third-party enclosure and plug that into your PC, then cross your fingers.
An external drive that doesn't show up in Device Manager as a USB mass storage device is most probably physically faulty. Logical damage would show up in Disk Management (partition damage) or My Computer (file system damage).