If the RP1 lights up but nothing is showing on the display, check if it still functions ok when you use it.
If the system is still functional, then it is likely that either the display cable has come loose or the display itself has failed and will need to be replaced.
Your nearest Digitech service center may be able to replace the display for you, or a good local electronics repair tech could repair it for you.
You could try a reset:
Power off the RP1, press and hold the Utility button, turn the RP1 on, then press Parameter Up whilst still holding the Utility button.
Let me know if that works?