The LED display doesn't stay showing any information. This is to conserve battery power. You can turn it back on and display the settings by hitting the top button on the back of the grip frame.
If the board itself does power off after it initially turns on, then the battery is probably dead or the grounding post is corroded. The grounding post was a problem spot on several older guns due to annodize and aluminum oxidization. It is the center mounting post where the screw fastens the board to the frame. You can fix this by running a tap into the screw hole and by sanding the top of the post to expose the metal. Given that the age of the marker is old and the battery was probably not charged for a long period of time, the battery plates inside are most likely oxidized and it probably doesn't have much capacity left. I would replace the pack with a new one. They can be built rather inexpensively these days, with much greater capacity than the original.
Except for the Automag in front, its usually the man behind the equipment that counts.