First question is, can you access the data on that HDD? If you can, proceed further:
Back-up the data on another media;
Wipe the HDD using a low-level format of your choice (I usually use killdisk);
Format the HDD for PC use (Windows needs NTFS or FAT, Linux-based OS don't need any specific formatting);
Optional: restore data to HDD.
If you can't access the data, try again to access it from a Linux-based PC (Ubuntu, for instance) and transfer the data to another media.
Finally, if you have no important data on it, just proceed with the wiping etc.
house70 is absolutely right! The priority thing for you now is to save your data back, if there is something really important data.
But, if you still cannot access the inner data, you could try to use a hard drive data recovery freeware here that is developed to cope with the data recovery problems for different reasons. It is really worth a shot.
One more thing, from now on, you should learn to save at least two copies of your important data on different drives in case of data loss.