The FAA has grounded all American flights, cancelling any flights that haven’t departed until further notice. According to FAA advisory, abrupt decision is due to a computer power outage. This outage effects what is known as the notice-to-air system, something that relays important, time sensitive importation to airplanes and pilots across the country.
As of around 8:30AM EST., the FAA announced that flights should be departing again shortly (after Newark and Atlanta airport resumed their schedule).