LCD stops display moments after starting

The LCD displays correctly five seconds or so then stops with keypad input. Both nano and uno behave the same. I assume this is a hardware problem but have little experience with these. I would appreciate your advise.

You could be correct, or wrong. I cannot see your wiring, or your code, so who knows.

Depends what you mean by "a hardware problem". The most obvious such is naïvely thinking you can actually power a project from "Vin" or the "barrel jack" and having the on-board regulator lacking proper heat-sinking, shut down when it overheats after a matter of seconds.

However if it is a response to keyboard input, it is far more likely a problem in your secret code. Do read the instructions for posting here. :grinning:

Does the LCD run longer if you don't press any button on the keypad?
Can it be that the keypad somehow shortens something if a button is pressen?

As missdeew already said, you have to provide more information. Without, how should someone be able to find out what's wrong?