You have probably corrupted your file system but most of your data should be intact. Try using RECUVA to recover your data (see free program listing on this forum).
I would then use WD Lifeguard diagnostics to checkout your disk. but NOT when the USB icon shows they are connected and certainly not when you are transferring data. Like Seavixen says eject first.
With WD disks like my "MyBook Essentials" they can go to sleep and you need to unplug and reconnect
Edit: I must have WD on the brain! As you say you have a Seagate. When sorted run the Seagate diagnostics.