my pc not turning on but motherboard led is?

hello, i was wondering if you could help me dove this situation. i’ve just finished building my pc and plugged all the cables in. but there’s a problem. only the LED on the motherboard will turn on, nothing else. i cant see to what seems to make this happen, it can’t be the psu because how would the led turn on? (the led was turned on by the 24 pin)