Moved Computer, Changed Monitor, now blank screen with flashing cursor

I am very much a Linux beginner.
I got my workstation (HP Z4 G4) running RHEL 8.7 with a 4K monitor.

Shipped the computer and tried to restart with a different monitor (maybe lower frame rate).
Computer starts, red hat picture lower center, then goes to a blank screen with a flashing cursor. No GUI running.
Can escape to control line mode with Ctr Alt F2, then login to computer, do command line things.
Can restart graphics mode and goes back to the same blank screen with a flashing cursor, using;
systemctl restart gdm

Could this be a monitor frame rate issue? what rate is the workstation using - how do I find out in command line? (I had issues with similar monitor before on Windows).

Or is this likely something else, due to shipping computer...

Thanks,

Paul