Why will my Xbox turn on then off instantly? Can someone help?

Here is a troubleshooting guide that may help with that issue:
https://support.xbox.com/en-US/xbox-one/console/console-turns-off-unexpectedly

This generic solution is not applicable here as the the device won't even turn on. It switches off automatically within fraction of seconds after turning on. So definitely it is not a power supply issue. Secondly, I turned it on after 3 days today. So
over heating scenario is also ruled out.

Hi!!

Check the steps marked on this link to see if anything helps you.

https://support.xbox.com/help/hardware-network/console/console-turns-off-unexpectedly

Tell us if there is any change!

try this https://support.xbox.com/en-US/xbox-one/console/reset-power-supply-solution

Please see the following pages, depending on your console type, to troubleshoot this problem. The most likely issue is a failing power supply or a temporary glitch with your power supply.

Please unplug the console and power supply (if you have one) for 10 seconds to reset it and then try to boot up again:

Xbox One Original:
https://support.xbox.com/en-US/xbox-one/console/reset-power-supply-solution

Xbox One S or X:
https://support.xbox.com/en-US/xbox-one/console/console-wont-power-on