Your result maybe if you removed the ext. HD before it properly dismounted. Anything along those lines will sooner or later cause an issue. The end result is that the ext. HD has become FAT corrupted or similar or possible physical damage. Now, you can't access it for whatever reason. If you can try accessing on another system and report what it states. Don't try different ports on your current PC, usually won't work. Please state fully just how all this happened.
As Robert offered find the "sticky note" on this forum to use whatever tools to see if they help. However, do this as a last resort, repeated attempts at data retrieval may causes deeper problems. Also, your ext. HD maker may have a FAQ or help to resolve this, visit their support website for model#.