Did it show up in Disk Utility? If it did, what exactly happened? Have you had problems before? Do you have any external hard drives that DO mount? Have you reset the open firmware (command+option+O+F at startup, then "reset all" ENTER)? It probably won't fix it, but it is worth a try.
As for Data Rescue, I only used the OS 9 version. It worked great, but you had to have open space on a different hard drive equal to or greater than the amount of data you want to save off the old drive. It also does not always work, especially if the drive has a hardware problem such that it can not be read. But if the problem is a directory problem, then it will generally be able to read and save your files.
The OS 9 version had a trial so to speak where you could save one file at a time to see if you liked it. You might try that (if it still works that way) to see if it can read and save your data. I know there is a TRY option for Data Rescue on their website, but I don't know what all it lets you do.