Not too sure if a faulty sensor would prevent the power ON signal but I will look around for any circuit diagrams e.t.c

Let do some basic checks:

Are you 100% sure that the power supply works?

Do you have a speaker connected so that you can check the error beeps?

What CPU is the system running (post exact model here) and are the "Dip" switches set correctly for it?

Have you tried turning the system on with only the basics connected
e.g. Take everything off the board, RAM - Video card ? e.t.c and try to turn it on, see whether the fans spin up.