Diagnosis
The battery is likely not making proper contact with the router's terminals. This model has a known issue with the battery latch mechanism.
How to Fix It
Remove the battery. Firmly press the black plastic battery latch on the tool with your thumb. You should feel a distinct click. Reinstall the battery. You should hear a solid click when it seats. The tool should power on.
If That Didn't Work
Inspect the gold battery terminals inside the router's base. Look for black carbon scoring or debris. Clean them with a dry cotton swab or pencil eraser. Do not use liquid cleaners.
Heads Up
Do not force the battery. If the latch feels mushy or broken, the tool base assembly needs replacement. Using it with a loose battery is a safety hazard.