I built my first PC yesterrday. All worked fine, loaded os & other software. Was able to use PC for several hours before switching it off. Went to bed (you needed that bit of info, right?) and tried to swith pc on ths morning - nothing except an led power light. PC was switched off and turned off at mains inlet, though the power cable was still attached.
Have tried to power up with only cpu/mobo power - same.