Most digital displays today are LCD.
The LCD display is "fed" it's signal via a conductive strip along the edge of the display that connects it to the circuit board generating the display information. If that conductive strip does not make good contact, one or more of the display segments will not receive their electrical signal, and your display will be missing pieces of the numbers.
As an EE, I've gone into such devices (when unplugged) and cleaned the contact area. Most often, when re-assembled, the display works again.
Unless you know a lot about electronics, you'd best stay out of the device and let a qualified technician fix it.