Network loss.  Reboot your CLS asap.

slow down there tonto net loss would cause loss for all agents. Check agent id are correct for those agents first, it may just be a mapping issue.

When I was born I was so suprised I didn't talk for 18 months

I'm far too familiar with the problem.  Any glitch in the network connection and the CLS stops sending consistent CTI data.  If there was a mapping problem then it woulden't have worked in the first place.  If you reboot you should be able to see if all your agents show up after about 10 minutes.

I agree with bigles up there that might be more to it than a simple reboot.  First try logging other agents into those phones, or log the agents that aren't working into other phones.  Try to work out if it is an issue with the agents or the extensions.

If it's the agents (i.e. doesn't work from any phone but other agents work from their phones) check the configuration of the users in Nice Administrator.  

If it's the extension check the extensions are in Avaya CT security group.

There is obviously a chance a reboot may fix the issue but I know for a couple of my sites that would require change control which could take up to a week to organise (two agents quality don't require an emergency change as the extensions are still recording)