Your board is probably fried
The only remedy is attempting to reset it, that may not work, and if so, you will need a new board, or a new gun.
Good Luck.
To reset your board (you will lose all personal settings)
Open the grip and press the 2 internal buttons at the same time... hold for 2 seconds and release. This will reset the board to factory specs. You will also lose all your personal settings, but the code will be cleared as well. It'll probably come back, but you might get lucky. This is what my tech told me to try before we replaced the board