Do you have a different Dell AC adapter you can use to test the pc?

a bad adapter could do this.

cpu is almost never the issue of no POST.

does power button turn white or blue at all after the single amber blink?

unfortunately i don't have another 90 watt power adapter to try, i have a 60W but that doesn't work, the power button just stays off totally after the 1 blink, then a little while after i see the hard drive light blinking

Take it to a computer repair store such as Best Buy in US where they may have a Dell adapter to test the pc

hi i did that yesterday the same issue happens, any other suggestions?

1 blink is not in any Dell amber dx code

https://www.dell.com/support/kbdoc/en-us/000126021/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators#2012_to_2020

(1) try RTC reset. https://www.dell.com/support/manuals/en-us/optiplex-5070-micro/opti5070_micro_sm/?guid=guid-f43ca142-3465-4f38-8fbe-2ad251ea73ff&lang=en-us

(2) check cpu socket pin since you swapped motherboard. Use a magnifying glass to look at the socket at 4 orthogonal angle closely after carefully extracting cpu. If you notice any bent pin, that is it.

Hi ok so i got a new motherboard and now it turns on but there is no video, no noticeable error codes either the light is normal white and doesn't blink or change to amber, i see some hard drive activity the keyboard and mouse shows signs of power , but don't get anything on the monitor

replace cmos battery of new mobo and clear its cmos settings (RTC reset).

I tried that but still no video, any other suggestions?

Re: i even bought a new motherboard

inspect mobo closely for bent pin

hi i checked there is no bent pins 

try remove all ssd.  you tried new mobo new ram different adapter.  only cpu and ssd are original

can you help me

1. my dell my dell OptiPlex 5070 computer when connected outlet power power supply ight blink once time after that the system is no ligate.