If I had a GigasetGS290 with a black screen in any of the modes I would do a Hard Reset. (I might try Soft Reset first just to check the effect.) I checked the manual, and (in a short search) found no reference to Soft Reset or Hard reset. The manual says for information further to that in the manual you are to contact their support service, in your country, seems to be available online or by telephone.
If the black screen is in say the Fastboot mode only I would assume I could use Factory Reset from within the phone and the Utility modes would all come good.
Update When I asked at Gigaset Service portal about Soft Reset I received this reply:
You have asked us how to do a soft reset on this model, first of all, you should remove the SIM card and SD card before resetting.
Also, we recommend you to back up your data beforehand, as all memory contents will be deleted.
In order to reset the phone, please follow next steps:
"Settings > System Settings > Backup & Reset.
and select “Reset to factory defaults”.
Or (depending on the software version)
Select Settings > System > Reset options > Delete all data (factory reset). > Restart phone.
We hope that you find this information helpful and useful regarding your inquiry, if not, please contact us back and we would be more than happy to provide additional information.