I upgraded my 6.0 PiAware by re-flashing the card to the latest Raspbian Linux 7.2 image and now can’t get a command prompt to re-enable ssh. This is on a RPi4. When connected to an external monitor the boot messages stop at various messages such as waiting for Plymouth to start or waiting for boot process to complete.
Try the other HDMI output on the Pi 4. The outputs aren’t mirrored (because of problems with that + plymouth) so only one gets the full output.
Failing that, if the boot process does not complete to the point that you have the (console) piaware status display visible, then you probably have a bad sdcard - try reflashing or try a new sdcard.
Once it boots to the piaware status display, you can get a login console on alt-f2.