The most likely cause is a safety lockout from the switch. Check for a locked trigger or a dead battery connection.
- Ensure the battery is a BL1815N, BL1820, BL1830, BL1840, or BL1850 cartridge. Others won't work.
- Firmly insert the battery into the saw. Listen for the click.
- Check the switch. It must be in the off-position before connecting the battery. If it was on when you inserted the battery, it won't start.
- Press the safety lock button and squeeze the trigger. You need to do both.
If that didn't work, inspect the switch itself. If the switch does not turn the saw on and off, it's dangerous and must be repaired. Do not use it.
Heads up: Prevent unintentional starting. Always ensure the switch is off before inserting or removing the battery.