Does it not work in both, windows and MAC? If that is the case it is just broken and if it still has warranty, get in touch with seagate or the vendor. Drives break often.
If there is data on the HD that must be recovered, and the device still has warranty, get in touch with a recovery agency before contacting seagate. They usually have contracts with the manufacturers so they can remove the HD from the USB case and recover data without voiding warranty. A good such agency is Gillware:
http://gillware.com/If there is no warranty left, you can try removing the drive from the USB case yourself, attach it as a 2nd HD internally to a PC, and if the HD is discovered by the BIOS, start your normal OS on that PC and check if you can access the HD. Now just copy the data off. If the OS can't access the HD, scan it with getdataback (get the correct version, probably it is FAT, as most external drives are formatted in FAT so it can be used by multiple OS's). If the tool sees the files you need, register it so you can copy the data from the disk.
http://runtime.orgIf the BIOS doesn't see the HD, it is bad and you won't be able to use it again, but again here a recovery agency might be able to recover the data (see the earlier Link I posted, or google for an agency closer to you).
If the disk can still be used with the MAC, but not with Windows, then there's nothing wrong with it and nothing to fix. Then the most likely issue is that it has been formatted to a MAC file-System, which isn't compatible with Windows. If you know the file-system, you can look online for a tool which you can install into Windows so you can read it. Paragon has such an "HFS+" file-system utility available...
https://www.paragon-software.com/products/home/
