The grid(s) could not be successfully accessed. This may be due to incorrectly specifying the paths or not having permission to access the data folders.
If the path and permissions are okay, the next possible source of the problem may be the result of missing required component files in the grid's folder. A valid grid must at minimum have dblbnd.adf, hdr.adf, and sta.adf files, as well as at least one pair of files in the format w00n00n.adf and w00n00nx.adf (where n is typically 1 but can be more for multitiled grids). If any of these files is missing from the grid folder, the grid is considered invalid.
Note that for integer grids, a vat.adf file is also usually present but is not necessary.
If all the required components are present, it is also possible that the files that contain the binary raster data (the w00n and w00nx files) are internally corrupted.
Check that you have correctly identified the dataset(s) and that you do have read permission. If this has been done and the problem continues, you should then determine whether the grid is a valid one. First try displaying it. If you cannot display it, check to see that all the required component files are present in the grid's folder. If there is no sta.adf file in the grid directory, try creating one with the Calculate Statistics tool. If its data files seem to have been corrupted, you may need to re-create the grid. Hopefully you have a backup copy that can be used or can run the process that created it again.