Why Permission Denied with NFS after reboot?

The problem is that the clients didn't realise that the nfs server went away, so they're still trying to access the filehandle that was originally created the previous time they mounted the file system.

Normally, rebooting the client is a sure way of making it remount the file systems. But if you don't want to do that, start by killing all processes that are trying to use the NFS file systems. After that, you can try a "lazy umount" with

umount -l

You might also try to remount the filesystem, using

mount -o remount 

Otherwise, the old file handles will timeout at some point, though I don't know how long it will take.

Once you've successfully gotten rid of the stale filehandles, remount the filesystems:

mount nobackup/dat