Fix cPanel email issue - emails not being received

We have a dedicated cpanel / whm server, emails have been working fine for years, but earlier today one of our catch all accounts stopped receiving any emails. We are still receiving email in other accounts on the server. A test message to another mail box on the same account as the problem account has come through fine, so it's just the catch all account that's the problem. Outgoing messages are working fine, it's just incoming. Our usual server admin hasn't been available this evening, he's had a quick look but not been able to resolve the problem, and we could do with this back up and running ASAP.