my greenworks 21 inch cordless mower suddenly stopped while i was mowing. now it won't restart, any ideas?

This is almost always the battery. It either ran out of charge or overheated and shut down.

Here's what you do:

1. First, let the mower and the battery cool down for at least 30 minutes. It has built-in thermal protection.
2. While you wait, check the area of operation. Make sure you're not on a slope greater than 15 degrees.
3. After it's cool, try charging the battery. Use the 40V charger. A fully drained battery will need a few hours.
4. Once charged, reinstall the battery firmly. Hold the safety start switch and pull the bail lever to try again.

If that didn't work: There might be a blockage. Turn the mower over (after removing the battery) and check for a heavy buildup of grass wrapped around the blade spindle. Clear any debris.

Heads up: Always mow when the grass is dry. Wet grass clogs the deck faster and strains the motor, which can cause this exact shutdown.