Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
Whats the full name brand and model of the device in question?
Thank you, ***** ***** understand what you meant about the black cylinder.
Ok, are we certain this isnt a monitor issue? Lets completely disconnect and reconnect the monitor.
Also, since you say the back lights up. lets try a pram reset
Shut down your Mac.Locate the following keys on the keyboard: Command (⌘), Option, P, and R.Turn on your Mac.Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.Hold these keys until the computer restarts and you hear the startup sound for a second time.Release the keys.
Now, will the mac turn on?
If not, when you try to turn it on if you hit the caps lock key, does it light up?
If yes, stop here and let me know. if the caps lock key doesnt light up, continue.
lets try removing and reseating your memory as follows https://support.apple.com/en-us/HT205044
Please thoroughly try each step, respond to each question Ive asked, and dont skip anything!
Hello again, I see you viewed my last reply but never responded. Did you still need help with this issue?
Please respond when you can so we can mark this issue as resolved or continue working on the problem.