TW update (zypper dup) has crashed two of my machines this morning

Looks OK and the (failing?) upgrade should not have broken it.
If your system actually boots to the (incomplete?) upgraded snapshot, you can open a VT (CTRL+ALT+F4), login as root and give “zypper dup” another chance at completing the upgrade.
Please check
https://forums.opensuse.org/t/lost-desktop-and-network-after-tw-plasma-6-update/173122
if you have no NetworkManager connection.