1st off, turn the jackpot key once and wiggle the displays' Molex connector.
(I do wish you would have shown us a photo or description of which display you have.
There are two different kinds for the S+ model)
Anyways, does anything come on in the "Coins Played" window when you wiggle the connector?
If not, you can try this...>>>
Tony's suggestion is good because your display works "part-time"....you MAY have a bad eprom in either the Reel or Game socket.
If you put in a different SP chip, the display should show a [67] or a [68] meaning you've changed the GAME prom.
To get it to "BOOT UP", you would then have to switch the power switch "OFF", then back "ON"....now it should go to [61].
At this point, you press and hold the test switch for about 2-3 seconds to get the display to show [61-1].
Now, close the door and turn the jackpot key once and after a few moments, the reels should spin from right to left and the "Insert Coin" display should light up.
This means you've had a successful "BOOT UP" sequence.
If you have a DBV, you will need to run a SET chip to enable it & set the denomination value of the coins that you'll be using.