The symptom is that KMail won't fetch mails for one of your IMAP accounts anymore, and if you open Akonadi Console (
akonadiconsole), it shows there "Connection established" for the resources in question, rather than "Ready" as it should say.
To fix this (until it recurrs), open the Akonadi Console (
akonadiconsole) and restart the akonadi server via "Server -> Restart Server" in the menu. It's also possible to restart the akonadi server from the command line.
This issue is related to KDE bug 257969, and as explained there in comment #8, it's a common symptom of several other issue reports. See there for more information if restarting the akonadi server did not help you.