If your root server hoster loves to integrate 10 year old harddisks with a history of 89 previous customers you’ll have to cope many times with disk failures. If you launch a shutdown -r now or reboot you’ll likely see Input/output error messages and nothing more.
How to boot? You might use your IPMI interface or some kind of hosters panel. If this is not available for any reason here’s how you might be able to force booting immediately:
# echo 1 > /proc/sys/kernel/sysrq
# echo b > /proc/sysrq-trigger
This works (as root) because you can trigger the Magic_SysRq_key concept also via the proc filesystem.