hema 7 navigator

Vertical lines on a display screen CAN indicate the connection from the circuit to the screen is NQR, not quite right.
Has the device been kissing the ground recently? that happening will dislodge the screen.
The screen may have a small, fine multi wire lead to it and it's connector to the circuit board may be loose/ dirty.
You wouldn't know any of this until you open the case for inspection.

If recently stored in cold frost areas, ie, inside the vehicle,ie, frost outside, the problem could be moisture condensation inside the case. The circuitry works on extremely small voltage levels and it moisture is present, just the moisture between components can be a path to confuse the system.
Opening the case and thoroughly drying it near a warm airflow source might restore the unit to working. The turn on not working is, to me, another indicator of moisture inside.
Dry it out and try again. No promises.
If possible place a small packet of silica Jel/Gel inside or keep the device in a bag which has the drier packet in there too.

No actual repair may be required, just maintenance of sorts.

I had that before.
On the back there is a reset button, very small, you need a pen or pencil to press it down.
Fixed it for me.
Cheers

I had the same after it fell off the suction mount. The reset button did the trick for me too, thankfully!