DIAGNOSIS The main PCB isn't getting power. The most common cause is the battery connection pins inside the compartment are stuck down.
FIX Open the battery cover. Look at the three spring-loaded pins inside. Press each one with your finger. They should pop up. If one is stuck, use a small flathead screwdriver to gently pry it upward. Reinstall the battery, making sure all three pins make contact.
IF THAT DIDN'T WORK The internal fuse on the mainboard may have blown from a short. This requires opening the case to check. It's a small, clear glass fuse near the battery terminals. If it's black inside, it's blown. Replace it with an identical 2A, 125V fuse.
HEADS UP Never use metal tools to scrape the battery terminals. You can short the board. Use a plastic spudger or toothpick.