Desktop black after new PSU install

Welcome to the forum, Soren. This is definitely a curious one. Since you can still see the mouse cursor, can you access task manager at all using CTRL + SHIFT + ESCAPE? If so, make sure "More Details" are expanded, then click File > Run New Task. In that new task, type "explorer.exe" and click OK. See if that causes your taskbar and icons to appear. If not, let's try disabling the App Readiness service.

While in Task Manager, navigate over to the Services tab. Click "Open Services". Scroll down until you see "App Readiness", double click the service, then change it's Startup Type to Disabled. Restart the system afterwards.

If this fails, then we should try to boot into safe mode to see if your desktop appears. Here is an article on how to do so: https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode. Make sure you follow the instructions for the Blank/Black Screen part, or the Sign In part if you can still access the sign in page.

If you can see your icons in safe mode, perform a clean boot and restart. Here is an article on how to perform a clean boot: https://www.microcenter.com/tech_center/article/9067/how-to-perform-a-clean-boot-in-windows-10.

Lastly, if that fails, it may be worth attempting a system restore. While I can't imagine anything changed with your software configuration when you swapped PSU's, it's possible that an update applied at shutdown before you powered off to make the swap. Here is an article on how to utilize System Restore: https://www.microcenter.com/tech_center/article/9025/how-to-run-a-system-restore-in-windows-10. Alternatively, if you have your data backed up, you can reinstall Windows using installation media, but that would be a last ditch effort, so I would start with the previous troubleshooting steps before resorting to that kind of resolution. If you need any assistance, let us know!