1. Go and check how much space is left.
2. Maybe there are many vps causing problem with server on the node.
3. Maybe you have installed badly coded addon/mod.
4. Maybe Your site is under ddos attack.
(check connections through the executing command below in ssh)
Code: 
netstat -ntu | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -n
And install http://deflate.medialayer.com/
5. Check the system processes and see whats causing it by entering "top" in shell.