I'm getting an error message

Yes

first lets try fixing the issue.
Hold down the windows key and press R to open the run menu
Or press start -> run

type cmd and press OK or enter.
In the cmd window, (black background) Type CHKDSK /F /X /R G:
Replace the G: with the correct drive letter, to find the correct drive letter open my computer from either desktop or from start menu, find your drive and see the drive letter in front of it, for example if the letter is H you would write the following

CHKDSK /F /X /R H: (press enter)

The scan will take a while.

After the scan is complete try opening your documents again.

If you are still unable to get to your files
I recommend file scavenger data recovery software
http://www.quetek.com/prod02.htm

Been using it for few years now myself.
Download the demo of the program and scan the drive with it and see if it is able to find your files, if it does find your files you can consider buying the program to restore all your files from the drive.

after all the important files are backed up into your computer, you need to format the external drive to get it back to use, right click on the drive and select format.