The 1st thing I would check are your tire pressures.(sounds strange, I know) , the ASR system sometimes will error out if your pressures are way off and then they subsequently will disable the throttle and won't allow you to go more than 20mph. When this occurs, your rpm's will race, but the car simply won't do anything....Other than 10-20 mph.

Check the wheels, tires, pressures etc. And then cycle the key. Worked for me on a 600SEL last week that I was sure had throttle problems, turned out the tires were overinflated to 50psi.

If the tranny goes into limp mode, it will only operate in 3rd gear (but ultimately you could reach a speed of 50mph).