I've just booted to find my software RAID 5 in Ubuntu not mounting. When trying to mount it gave me an NFS error (which was confusing). I ran fsck on /dev/md0 and my screen scrolled with fixes for about an hour. It claimed to be complete, however I've mounted it and the folder structure is empty. It just has a lost+found folder containing hundreds of files like the screenshot below:
Looks like the filesystem was hosed and the fsck didn't fully repair it.
At this point I'd be tempted to check the logs to see if the disks are all physically working (noises? SMART status? errors in the logs regarding resets? etc.) and restore from backup rather than spend more time trying to straighten out the results of the fsck.