No reason to assume just because a drive shows as unmountable that anything has been lost. Handling of unmountable drives is covered here in the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page. The Unraid OS->Manual section in particular covers most features of the current Unraid release.
the question, however, is why the drives went unmountable. Probably a good idea to attach your system’s diagnostics zip file to your next post in this thread to see if we can spot anything.