Falcon 9 was set to lift off from Cape Canaveral on May 19, to take half a ton of provisions to the ISS, and was meant to become the first private-made vessel to perform such a task.
The countdown was almost complete and the engine ignition sequence almost started, when the on-board computers halted the process automatically.
A report on the SpaceX website
states, "Today, SpaceX aborted the launch of the Falcon 9 rocket and Dragon spacecraft. Due to the instantaneous launch window, we are not able to recycle and re-attempt the launch today.
Early data indicates that the flight computer detected slightly high combustion chamber pressure on engine 5, which prompted the computer to abort the countdown. We are reviewing the data."