External hard drive excel files

Yesterday, I backed up many of my excel files and office files on an external hard drive which had help most of my important files. However, I didn’t know what had happened to this external drive. When I tried to access these stored file on my laptop, a message said it needed to be formatted at first. Is it virus-infected or getting other problems? Please, help me rescue these files. Thank you very much!

Have you tried this external hard drive on another computer? Get the same problems?
As I know, the “not formatted error” problem often happens for many reasons, like the virus-infection, RAW file system, power surge and even computer system problems, etc. But, it often can be easily fixed by re-formatting.
So, don’t worry! In your case, just firstly rescue your wanted drive files by using some data recovery software, like Recuva, iCare Data Recovery Free and Power Data Recovery, etc.
And then, format this drive to fix its possible problems.
Remember to make data backups all the time in the future in case of similar data loss problems.

That could be caused by virus-infection. But, since you do not offer enough information, I am also not sure. But, as to your inaccessible drive files, I think I may give you some clues.

Here are some steps for you to go on:
1). Stop using this external hard drive in case that anything else will overwrite your files inside.
2). Connect external hard drive to computer and use a data recovery tool to rescue all your wanted data back. There are many similar data recovery tools online that can help you. Just search one over the internet.
3). Save and back up restored files on another drive in case of farther data loss.
Also learn a lesson to make data backups all the time in the future.

Here is an article that can help you rescue your wanted files back in details:
not-formatted.blogspot.com/2013/02/recover-data-from-not-formatted.html