What Messenger version are you using? It happens to me from time to time...
Do you have the Plus POP e-mail check feature enabled? (Plus! > Preferences > POP3 Email).
Checking for E-mail - by Yuki on 01-23-2005 at 08:21 PM RE: Checking for E-mail - by lopardo on 01-23-2005 at 08:38 PM RE: Checking for E-mail - by Yuki on 01-23-2005 at 10:50 PM