Firstly, I'd like to thank you for your replies, they mean a lot to me.
Took it to a technician who claims that it's power unit or something is broken and it's not receiving power.
Depending on the laptop this could be a cheap part to replace.
It's relatively cheap to replace, about €60 for both the replacement part and the labor. It's a refurbished laptop, not of too much value, but gets the job done.
While I wouldn't really bother about the rest of my files, I'd rather not have someone recovering my wallets this way.
How about passwords? Creating a new wallet and moving all your coins is quite easy, but your browser might have access to many accounts too.
That's right, that's something I forgot about, but the most important thing there is my wallets, therefore, I completely forgot about anything else but them.
Is there an actual way to permanently delete such files from the HDD, forever?
There's a software
shred and there's the hardware
shredder.
I would prefer not to trust anyone with my data, so I would have removed the drive before sending the laptop to a repair guy.
Stupid of me for not thinking it earlier, tomorrow is not a working day here and pretty much rushed to get it somewhere to get it fixed, since I used it for most of my daily and work stuff. He's a quite known and reputable guy, so I believe nothing will be done in the first place, I'll try not to be paranoid about it. Definitely took my lesson in case something like this ever happens again.
If I'm to sell it, I'll either replace the HDD with an old one I have here or use one of the recommended software to overwrite the files.