Why is the boot button turning blue but then turns orange

A failing power supply can cause exactly this: quick blue light, then amber, and no startup. If you have access to another compatible power supply, try swapping it. Also, the "low power mode" message that you're seeing is most likely coming from the monitor going into sleep mode after not receiving an input from the PC. If the power light is blinking at all, try googling the model name of the PC and the number of power button blinks that it makes. That will usually lead you in the right direction. Hope this helps!