No picture to tv

And suddenly while everything was working very well I have no picture on the television.
What could have happened.
And what I can try so bring back the picture?
My friend’s setup is ondroid xu4 and the dietpi operation system.
He has installed nextcloud with a ssd and he doesnt dont want to lose his stored data.
Thanks for any help to recover.

Hi,

Does the XU4 have a heartbeat (flashing LED)?

Hi thanks for the replay and sorry for my delay.
Yes, the XU4 runs normally everything works but there is no picture on tv.
Something has been changed to boot section .We still searching .

Yep,

Have you tried edid/non-edid modes?