Denise's external hard drive can't be read by her Mac. How can she recover it? Leo says that there are disc recovery tools that could help Denise. One is called SpinRite, but it's a bit expensive and is difficult to use on a Mac. AlSoft's Disc Warrior is another good option, but it may not be the best option because it can only fix soft failures. Another one is ProSoft Data Rescue 3, which is $80. A hardware failure may have occurred and if that's the case, the hard drive may need to be taken in to a service like DriveSavers to get the data off, and that's very expensive.
Doctor Mom in the chatroom says to double check all the connectors. She should try using a different computer, and a different USB cable, just in case.
The other problem may be that if she upgraded to macOS High Sierra, Apple has changed their file system and plugging in that USB drive may not be readable without it being converted.
This is a good example of why backups are important.