It is difficult to read your image, but my best guess is that you were running
1.9-q-20230310268292-stable-GS290 so Android 10 (Q), 10 March 2023.
I am hoping that you will find that you can reach fastboot mode and do not need to read on.
Several years ago there was a run of forum threads like “GS290 with black screen”, you may have seen them in your searches.
GS290 black screen bookmarks
The first link here may be the one with the closest to a user documented solution Stuck on fastboot flashing unlock(Now Black screen)
Black screen, don't start
Bricked GS290 (#3429) · Issues · e / Backlog · GitLab
Installing /e/OS after removing UbuntuTouch on the GS290
This thread gives some insight to the position with TWRP Gigaset GS290, how to root and install ZIP files?