DIAGNOSIS A completely dead mower after storage usually points to two things: deeply discharged batteries, or a poor connection where the main battery harness plugs into the mower's electronics.
FIX Start with the batteries. 1) Remove all battery packs from the mower. 2) Put each one on the EGO charger individually. If a battery was left in the mower all winter, it may have self-discharged below the charger's recognition voltage. Leave it on the charger for at least an hour—sometimes they need a long wake-up period before the charger light even comes on. 3) Once all batteries show a full charge on their own indicators, reinstall them in the mower, making sure each one clicks firmly into place. 4) Try turning the key.
IF THAT DIDN'T WORK If the batteries are known good and charged, check the main 40-amp or 60-amp fuse in the battery compartment. It's the big one, usually in a holder by itself. A fuse can blow if there was a minor power surge or if something was accidentally shorted during storage. Replace it with an identical fuse.
HEADS UP For long-term storage, EGO recommends storing the batteries separately from the mower, in a cool, dry place, at around a 50% charge. Leaving them plugged in all winter is the most common cause of this problem.