In your case, you have no choice but to format your hard disk to make it workable as before.
However, if you worry a lot about your data inside it, you should try a hard drive recovery freeware, which I have tried to recover my files after receiving a “not formatted error”, just like yours. It is really worth trying. Note for you:
1. Do not format it until you’ve restored all your data.
2. Do not store any new file on this drive to avoid the original data overwritten by new data, which can make your data gone permanently.
3. Do not save the recovered data on the same drive in case of recovery failure.
4. Do not forget to format this drive after the recovery process.
5. Do not forget to back up your important files as possible as you can in the future.