1 hour ago I did reboot my box for the first time...and it does not come back online. I did already send a ticket hoping they will help me. I know there are several reasons for this... but which ones are the most common?

this may happen, if server goes to fcsk or server is not configured for clean boot.

tell your host to check weather it comes online properly or not.

whenever i higher new server the first thing i do with it, is reboot.

and wait and watch weather it comes online properly or not.

then i shutdown it to check weather my host is monitoring it or not. ;)

but just in the beginning when i don't have any clients on it. :D

Mahinder, that's excellent advice. It's possible that the machine was never properly configured to survive a reboot, if you haven't tested it.

For a remote machine it's probably usually as the result of a network card driver. In that case the machine may be up, you just can't get to it.

Of course there's the leaving a boot floppy or CD in the machine. :)