The most common reasons are the 100K digit driver resistor has failed or cold solder connection to pins or panel.
I was working on my Power Play game and after reflowing solder to the panels I had a couple of digits out on two of my panels. In both cases it turned out to be a failed 100k resistor. BigDaddy has a display rebuild kit that comes with a new set of 1/2 watt 100k resistors.