Hi Marvalitos and welcome to the forum
Sadly, if the PC is not turning on after following the above advice, it probably means that you have a more serious problem.
In case the system still refuses to cooperate, I would start with a check of the PSU. You could use a multimeter in order to check both the 5 and 12 V line of the PSU (you can find video guides on the web as seeing how its done would probably be better than reading it). Another option would be to check how it works in a different system that has similar power requirements.
Another option would be to introduce a working PSU to our system and to see how it works with it.
I would also connect the HDD to a different PC (as a secondary drive) and would make a backup as it also good to have one. After that, run a diagnostics test and post the SMART status here. Make sure to hide the SN of the drive.
Keep us posted,
Frost_WD