I'm guessing it's similar to this: https://discuss.kde.org/t/plasma-doesnt-start-when-external-monitors-are-plugged-in/17350

You're probably on a laptop and have multiple GPUs enabled at the same time. If not, you're defining Nvidia drivers with something trying to be smart about when to enable the actual outputs or not.

You can either make sure to have the display turned on, or make it super clear to the drivers to always enable a display regardless of whether it detects a display.