You can follow these steps to resolve this issue. :
1. Open a SSH Connection to port 22
2. Login as root
3. Run the following commands (one at a time)
/scripts/restartsrv_imap
/scripts/restartsrv_courier
/scripts/restartsrv_cppop
The above commands will restart the IMAP server as well as some other vital services that are needed to access you web mail function on your dedicated server.