How do I fix a left crank that has come off of my Pro Form 4.0 RT Stationary Bike?
As mentioned in comments the crank arm is wrecked and needs to be replaced. Once the square taper hole in the crank starts to deform (and yours is quite bad) it's impossible to get them to stay on tight and the problem gets progressively worse.
If the axle square taper is undamaged you should be able to simply purchase replacement crank arms. The square taper interface is standardized. There are a a few complications for someone not used or equipped for working on bikes.

There are two similar square taper interface standards JIS and ISO. The angle is the same but JIS is slightly longer. The vast majority of bike components use JIS, but it's worth checking. A JIS crank will bottom out on an ISO taper.

The Pro Form 4.0 RT stationary bike has all the drive stuff hidden away internally and used two 'left hand' cranks. If you wanted matching cranks you have to buy two left hand ones.

To get the right hand crank off off you need a special crank puller tool. Video shows how they work but there are various types available.

The retailing bolts need to be tightened quite hard (~30NM). You really need a torque wrench to make sure you get them done up tight enough and avoid repeating the problem.