try replace cmos battery and reset cmos jumper (RTCRST) on motherboard.  you have a No video situation which can mean No POST if video cable connection and monitor are both good.

see p.15 for jumper setting https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-755_user%27s%20guide_en-us.pdf

solid amber power button = dead power supply or dead motherboard.

5vsb is working but nothing else is.