You will receive this error if Cakewalk cannot locate the audio data associated with the .WRK file you're opening.

In Cakewalk, go to Options | Audio | Advanced. Take a look at the Data Directory field at the top. The path you see is the location where Cakewalk looks for your
audio data. Mark this path down, and close Cakewalk.

Now choose Start | Find | Files or Folders. Type *.WA~, and make sure Windows is looking in My Computer. Click Find Now, and you'll see a list of .WA~ files appear
in the window below. Take a note of their locations. If the path of these .WA~ files is not the same as the path you saw in Cakewalk, you'll need to reconcile that. The
easiest way is to use Windows Explorer to simply move all the .WA~ files into the proper location. Once done, your files should open properly.
(If the Find | Files of Folders does not come up with any .WA~ files, they are most likely lost from the hard drive, and you'll need to re-create your files. )

If the .WA~ files are found and are in the correct location but you still receive the error, there is a way to manually recreate your files. Open a file that gives you the
error message. Choose Insert | Wave File, and navigate to the location of the .WA~ files. In the Name field type *.*, and hit Enter. You'll now see all of the .WA~
files listed. You may be able to audition them using the Play button, or you can tell which ones are which by the creation date. Double click on the files to import them
into Cakewalk. Once they're all in, you'll need to rearrange them properly, then resave the file.