Every once in a while discover (KDE's update manager) will simply freeze and stop fetching updates.

Restarting Discover has no effect. Manually running pkcon update also freezes with no error message or timeout.

Fortunately apt is helpful enough to tell you that a lock is held.

sudo apt update
Reading package lists... Done
E: Could not get lock /var/lib/apt/lists/lock. It is held by process 2833 (packagekitd)
N: Be aware that removing the lock file is not a solution and may break your system.
E: Unable to lock directory /var/lib/apt/lists/

So to fix the Discover updates you need to restart the packagekit service.

sudo service packagekit restart

Then Discover/apt/pkcon will start working again.