It sounds like you may have accidentally set the contrast to a very low setting. It might be difficult to adjust without being able to see the words on the screen, but you should be able to do it. Hold down the Looper Controls button for a few seconds to enter setup mode, and then in FX module 4, turn the knob in the upper right hand corner, and you should see the letters start to reappear.
Thanks for your reply! I tried it out, and I see how my display turns tu black but, no letters. The stompboxes works and everything. But I don´t see the information. It actually worked after a while but then it happened again. Should I get a midi usb cable and restarted? I got the pedalboard last week and I could do much with it.