I recently upgraded to Kubuntu 18.4 LTS and things were going okay. Got all my usual stuff installed. However, when I am prompted for updates and select them to be installed, it just sits there. I cannot close the window or cancel the updates, or shut the box down. It warns me it cannot as there are tasks that need to be done. Its pretty frustrating as I cannot even use Ksysguard to shut down safely and need to hard kill the box. Any ideas ?

Can you select them one at a time or use command line dnf to select one at a time?

I thought about that to isolate a possible culprit update
but haven't tried it yet. Need to kill the box first.
My laptop has the same version and wants to do updates
but I would prefer to know whats wrong first.

Just tried to reboot and got a message that it was blocked
by/usr/bin/plasma-discover

I killed and started it back up and selected ONE update
and I think it installed. Then the update prompt went away.
There are stil 2 more to go , but I cannot find a way to view
the updates. I cannot find a way to install Muon update manager
to do that.

have you tried waiting it out?

may i ask about your hardware specs, esp. CPU, RAM and partition space?

is this a full reinstall or a dist-upgrade?

Apparently plasma-discover obsoletes muon-updater:
https://launchpad.net/ubuntu/+source/plasma-discover
https://userbase.kde.org/Discover

Since I'm not using it, I'm guessing plasma-discover is working as a background task that needs time to initialize after you did your "recent" update to 18.04. How "recent" exactly? In order to use more conventional installation/update tools like apt or aptitude, plasma-discover may need to be disabled or stopped, maybe using systemd.

I tried waiting it out yes. After that one update it is not prompting
for any more. I installed 18.4 from a disc, overwriting the old
Windows10 application.
And yes it seems like Plasma-Discover has replaced Muon.
I miss Muon...

^ hardware specs please.
KDE is not exactly lightweight...

You can still do updates through Muon if you want. It's just that discover locks the apt database while it's doing its auto-check. Once discover lets it go you can use muon or update from the command line. Also, since the kubuntu 18.04 release, there have been several updates on discover as it was pretty buggy. I ran into the same problems you did when using discover to update after my initial install. Don't know if they corrected the problems with discover as I usually just kill the discover process if necessary and update from the command line. The good news is that after several updates to discover, the application does not seem to lock the apt database for as long a time and I usually no longer have to kill discover before updating from the command line.

Edit: I don't know if muon is installed by default but it is definitely available as is synaptic for that matter. To install run:

I didnt see Muon in the Discover apps menu . The box is 6 gigs
and was updating fine for the past few weeks/months ?
Its not a real recent upgrade. For CPU and other info
I would have to dig for it as I have 4 Linux boxes
and I cannot recall the data of them all. I can say
the partitioning was auto done at install.

Huh. I ran the install muon in the terminal and
it told me its already the newest version 4:5.8.0-0ubuntu1
0 upgraded 0 newly installed 0 to remove and 0 not upgraded....

Wondering if it is referring to the muon package manager
which IS installed, but not the muon update manager..

Okay. In the muon package manager only the package manager itself is installed.
there are 3 other muon packages available.
muon-discover transitional package ( muon-discover> plasma-discover )
muon-notifier transitional package ( muon-notifier> plasma-discover )
muon-updater transitional package ( muon-updater> plasma discover )

I also have the same 18.4 Kubuntu on my
laptop and it updates just fine for whatever thats worth.

I have just the muon package installed. On mine it's found in the applications menu under System.

Like I said, ONLY the Muon package manager is installed.
NOT the Muon UPDATE manager. I have the Muon update manager
on two other boxes but neither are 18.4. They are 14.4 and 16.4
versions of Kubuntu.

As for Hardware the box in question has an AMD A4-5000 APU with Radeon HD graphics.
Memory is 6144 MB DDR3 / 1333 mHz / single channel.
Dont know about the partitioning as it was doem automatically on install using the entire disc.