Ubuntu software will not load after new install of LTS 20.04

I have just installed LTS 20.04 on a new computer.
I immediately updated with Software Updater
I installed Brave browser using Terminal.
I realised I could have done that through Ubuntu Software when I explored it.
I noticed that 6 updates were pending on Ubuntu Software and tried to install them.
I got an error message that Snap could not update because it was set to auto-update. The same message for the other 5 (whose names I can't remember).
I closed Ubuntu Software and now it won't reopen.
Software Updater says the system is up to date.
Rebooting doesn't help.
The folders in Snap are all empty (Canonical Livepatch and Snap-store).
Mozilla and Brave are working well, but I can't download other software through Ubuntu Software and am worried this is because I used Terminal using commands from https://brave.com/linux/#release-channel-installation. Perhaps another possibility is that there is some incompatibility between Software Updater and the update function on Ubuntu Software.