I kept one of these for my cpu and one for my gpu on my desktop for sometime when suddenly one day a while back they were gone. It is still in the list of available plasmoids but there is no icon for it displayed there. If I try to replace it on the desktop it just drags an empty silouhette which disappears when I release the mouse and no plasmoid appears.
It would be nice to be able to get it working again.
Which Plasmoid is it and have you looked for others? I wonder if something changed in Plasma as of recent.
Thermal Monitor Plasmoid is the name of it and I don’t see any others that do the same thing in the list, under get more plasmoids, or in the package manager.
It doesn’t look to be receiving much in the way of updates, but perhaps try “Thermal Monitor” from the KDE Store https://store.kde.org/p/998915/
I removed and reinstalled it with add>get new remove widgets and it’s still the same.
I can add it to the panel here with no problem on this old system:
Operating System: openSUSE Tumbleweed 20200107
KDE Plasma Version: 5.17.4
KDE Frameworks Version: 5.65.0
Qt Version: 5.13.1
Kernel Version: 5.3.12-2-default
OS Type: 64-bit
Processors: 2 × AMD Athlon™ 64 X2 Dual Core Processor 5600+
and set it to display the two CPU core temperatures…
https://paste.opensuse.org/view/raw/55b7cf4e
… but I seriously doubt the accuracy of the figures 
Are you able to add it to a new user?
I must pay more attention …
Sorry, I’ve just reread your first post and see you’re adding it to the desktop, not the panel, and…
Indeed, when I try that, I’m seeing the same as you report.
I guess you could try adding a bug report over at the KDE Store, but as I wrote earler, it doesn’t look as if that plasmoid has received any attention for quite a while.
Yeah, ok.
Mine was very accurate. Sorry I lost it.
You’re still able to place it in the panel though, as in the screenshot I posted https://paste.opensuse.org/view/raw/55b7cf4e
Perhaps you could use that as an interim measure…
It is a thought. Thanks for that.
Huh. Whaddya know. Just fyi it doesn’t add to panel either.
Strange… Works here OK, as can be seen from the screenshot in post #9
Have you tried it with a new user to eliminate any problem with (possibly) “~/.config/plasma-org.kde.plasma.desktop-appletsrc”, or the user profile in general.
Yeah, that’s a good idea. I forgot about that. I’ll try that and let you know what happens.
Interesting. It wasn’t in the widget list as a different user so I tried to add it with the download new widget function. It was in that list but when I tried to install it said it couldn’t download. File not found.
Transient problem maybe? That (Add Widgets… -> Get new widgets -> Download new plasma widgets) was the way I added it back on 2020/01/10 when I tested it. I’ve just looked on the KDE Store and it’s still there. Don’t know I’m afraid 
…