scontrol show node nodename to see its status.
If node is drained, sudo scontrol update NodeName=nodename State=down Reason=undrain
Then sudo scontrol update NodeName=nodename State=resume or State=idle.
If node is down:
ping nodename to see if it can respond.
You can try sudo scontrol update NodeName=nodename State=resume but it may not work.
To reboot the node, ssh nodename then login as root with sudo -i, then stop and start slurm as follows:
/usr/sbin/slurmd stop
/usr/sbin/slurmd startclean