DIAGNOSIS The motor shaft or the blade adapter is seized, usually because something (like a thick stick or rock) jammed the blade hard enough to bend the motor shaft or weld the blade bolt in place. It's a serious mechanical lock.
FIX This requires disassembly. Remove the battery. Tip the mower, remove the blade bolt (it's on tight, might need a breaker bar), and take the blade off. Now try to turn the motor shaft (the silver stub the blade attaches to) with a wrench. If it still won't turn, the brushless motor's internal bearings are seized. That means a motor replacement.
IF THAT DIDN'T WORK If the motor shaft turns freely once the blade is off, then the blade itself was bent and binding against the deck. Inspect the blade for a major bend. Even a slight bend can cause it to hit the mower housing when installed.
HEADS UP If you hit a solid object, STOP immediately. Don't just keep trying to run it. That instant shock load is what causes this kind of damage. Always clear the yard of sticks and debris first.