The first thing to do is with it connected, start up Disk Utility from the Utility folder in the Application folder on your computers hard drive. With Disk Utility running see if it shows on the left side. If it does then run disk first aid and tell it to repair. When it is done it will let you know if it fixed it or if you still need to do more. Usually a program such as TechTool Pro would help at this point.
If it doesn't show up in the disk Utility window then you might have a more serious hardware issue or if you have it go straight to TechTool Pro and see what happens.
Bad news whenever a mounted drive is unplugged. Can corrupt drive and system.