[Related documentation: PAC LED lights]
If your PAC does not boot, but no software changes have been made to it, check the main LEDs next to the power input. If they show as solid blue, solid white and solid red from top to bottom, the issue may be that the SD card has come loose.
This light pattern, in addition to two audible beeps, indicates that no boot device has been found. The Installation Guide entry for this pattern reads "A bootable device has not yet been detected."
To fix this, power down the unit and remove the cover plate on its top side.
Then, click the SD card out of the slot and click it back in. It will be easy to tell when the SD card has clicked in. Avoid removing the SD card from its slot if possible because it is easy to drop it into the PAC's chassis.
This kind of problem is a rare occurrence, but it can sometimes happen if a PAC is installed in a machine and then shipped over a long distance. If there are serious enough bumps during the shipping process, the SD card can begin to loosen. Such a problem will almost never occur when a machine is installed and stationary unless there is serious vibration in the control cabinet.