@Sujon: Does the Power button feel stuck in place at all? Although it requires a functioning Power button, can you try pressing the Volume Down and Power buttons together for around 10 seconds to see if you're able to boot up the phone in this manner? If it remains unresponsive then please reach out to our Help Desk via the link below, where an advisor will be able to assist you further and arrange any necessary repairs.
Hey people "you can get in the download mode holding both volume buttons and then plug the usbe alredy plugged in the pc . now cancel pressing down button and it restart your phone". This is the solution from Eriktar at this link https://forum.xda-developers.com/t/galaxy-a7-2018-turn-on-without-power-button.3939676/. It worked great for me hopes this helps you 
