DIAGNOSIS The self-propel drive belt has either snapped or jumped off its pulley. It's a wear item, and tall, thick grass can put enough strain on it to break it. The loose lever feeling is the dead giveaway—there's no tension because the belt isn't connected.
FIX You'll need to get underneath. Remove the battery. Tip the mower back (rest it on the handle, air filter up). You'll see a black rubber belt connecting the self-propel gearbox to the rear wheels. If it's just dangling, you can try to reroute it onto the pulleys following the diagram molded into the mower deck. If it's broken or shredded, you need a new belt (Ryobi part number 514289004).
IF THAT DIDN'T WORK If the belt is intact and on the pulleys but still no go, the issue could be internal to the gearbox or the drive control cable came disconnected. Check where the cable attaches to the lever mechanism on the handle—if it's detached, you can reconnect it.
HEADS UP When replacing the belt, make sure the mower is in the 'fastest' speed setting (lever fully forward) to give you the most slack to work with. It's a tight fit and can be a bit of a wrestling match.