The option wyrdoak is talking about is already set to never.
I'm not talking about monitor going to sleep but about the OS going to sleep (it's a home server). Most of the time the screen is off anyway.
Just to make it clear - 2 days ago the system was shutdown when I woke up in the morning. Since then it still up. As this is a new installation I assumed it wasn't a crash. The hardware is OK (This computer was running Mint 11 KDE which is now out of support). There was no power outage at that time. So I'm assuming the system went to sleep.