yesterday I shut down a server. When I tried to start it again, it wouldn't come up.
Error: storage is not online.
All other VM's are still running.
How to fix this?
The version you are running is not supported, it's EOL, not just Proxmox VE, but Debian Stretch (it's the base) as well: https://forum.proxmox.com/threads/proxmox-ve-support-lifecycle.35755/
So if you really want this system to keep running you should prepare an upgrade to version 6, instead of just focusing on fixing the question marks.
Fix the San connection, it indicates there is an issue.
Then upgrade.
I have had it in the past that issues only manifest themselves when a VM is turned off. That was esxi though...
