Dell Optiplex 9020 MT solid white light but video output, keyboard and mouse is not working

That is a very powerful video card that probably needs a bigger power supply. What model video card? Check the requirements for power at the manufacturer's site. What psu do you have now? What do you mean by swapping ram?

I'm using this desktop from last 5 years i upgrade it 3 years ago and in this 3 years it's working fine.

I'm using GeForce GTX 1050Ti gpu

DC maximum output power is 290w

Thanks for your assistance

75W video card has overloaded 290W power supply. Hard drive may also be damaged now.

Optional 365W unit is required OR EVGA 500W with Moddiy CAB 359

https://www.moddiy.com/products/Dell-OptiPlex-9020-PSU-Main-Power-24-Pin-to-8-Pin-Adapter-Cable-30cm.html

https://www.amazon.com/EVGA-100-BR-0500-K1-Bronze-Power-Supply/dp/B07DTMXD83/ 

DELL 365w
7VK45
TM143

If it hasn't been done within the last 5 years, replace the size 2032 CMOS battery.  A bad battery will cause all sorts of seemingly unrelated bizarre problems.

Mind if I ask? - How is it known keyboard and mouse aren't working when there's no display?

With the PC off, if you push the test button on the PSU, does it give you the green light?

It's not unusual the PSU to start going out, or go out, on a somewhat older machine.

Screengrab is from the Optiplex 9020 Tech Guide.  It supports 75w GPU's on 290w PSU.

I still recommend replacing the battery first.  Cheap fix even if it doesn't work.  If you replace the PSU, it can't to go with a higher watt PSU as speedstep suggested.

I meant to say:  ...it can't hurt to go with a higher watt PSU as speedstep suggested.

Most grocery stores have Duracell DL2032 batteries.

https://www.amazon.com/dp/B07GN8ZYZR

Blood sugar machines use the same batteries.

They may be in the back and say for medical devices but they are the same.

This wont fix a bad power supply however.

Shutting down by itself is overheating or over voltage or worse.

Yep i replaced cmos battery but still facing the same issue.

I also replace the ram and also replace my GPU with my friend GPU but still i'm getting the same problem.

I put my ram and GPU on my friend computer same model Dell Optiplex 9020 MT.

My GPU and Rams are working fine on my friend PC.

Because there is no LED light from keyboard and mouse from which i think my keyboard and mouse is not working.

But when i put my one ram of 8gb my computer boots and started i install the fresh windows 10 and i use it for next 4 hours ( in which i installed and play GTA V ) 

But when i shutdown my PC and start it after 2 hours again i got the same issue no video output and no keyboard and mouse led light.

When i put 8gb ram again my PC starts.

After shutdown i need to remove or insert one ram to start my PC.

There are 4 ram slots i put ram in another 2 slots but after shutdown i'm facing the same issue.

If there are 2 rams i have to remove one and after shutdown to start my PC i have to insert 2nd ram again.

To start my PC i have to remove or insert ram again and again.

YES green light is coming from PSU when i push the test button my GPU and both other fan runs for some seconds and there is green light.

Sorry for my bad english and thanks for assistance....!

I'd try the RAM in slots 1 & 3 and/or 2 & 4.

If problem still persists, it makes me wonder if it's PSU or MB.  It almost sounds like an MB problem with removing and inserting RAM making it work.

Also, are you able to try it without the GPU card?