DIAGNOSIS This usually points to a safety switch interruption. The M18 saw has two: the main trigger lock (the yellow slide) and the blade guard/riving knife interlock. If either isn't fully engaged correctly, the saw gets no power.
FIX Follow the safety chain. 1) Ensure the yellow trigger lockout switch is fully depressed (you should hear/feel a click) when you pull the trigger. 2) Check the riving knife and blade guard assembly. Is it fully installed and locked into its lowest position? The saw won't run if it's in the raised 'guard removal' position or not fully seated. Try removing the blade guard entirely and reinstalling the riving knife by itself, making sure it clicks into place. 3) Try a second, known-good M18 battery to rule out a rare battery communication fault.
IF THAT DIDN'T WORK Inspect the trigger area for packed sawdust. Sometimes fine dust can jam the mechanical lockout switch. Blow it out thoroughly with compressed air. If it's still dead, the electronic switch in the handle may have failed, which is a job for a service center.
HEADS UP Always start troubleshooting with the safety systems. They're designed to prevent operation if things aren't right, and that's a good thing. Forcing or bypassing them is extremely dangerous and can ruin the tool.