It's hard to tell without taking a look at your VPS itself. What you could try is to login to SSH and enter:
service cpanel restart
Then see whether it reports that all parts are started successfully (it's no problem if things like interchange show up as failed - because that would happen if interchange is simply not being used), and then see whether cPanel still shows a blank page.
If it does still show a blank page, it's not likely that ram is the problem since the cpanel service script showed that the services started successfully. In that case it's most likely that there's some kind of firewall problem, which would be blocking things in WHM. To see whether that's the case, login to SSH and enter:
This flushes the iptables rules (which are used by a software firewall). Now try to load WHM. If everything works properly now, then there's a problem with your firewall rules - which you would either have to disable or reconfigure.
If at this point it still doesn't work, it could be a problem with cPanel itself. You could then try:
This will upgrade cPanel to the latest available version and will force everything to be reinstalled due to the "-f" parameter.
If at this point it still doesn't work it'll be best to have someone look at the configuration of your VPS - best to ask your provider.