The fault light and reason would need to be determined via a bite check. Possible overspeed condition, oil pressure etc (if it even did light off?). If in flt common for cold soak, no light off and/or overtemp at alt, on gnd less common, but does happen.
The switch has nothing to do with it (if it in fact started the sequence?). The reason you hold it in "ön" for a couple of seconds is to allow the FADEC to power up (it needs to be powered to allow the start sequence, if not going to start will do nothing).
Select "on" and watch the EGT gauge, when it flicks, you have APU FADEC power.