XPS 13 9350, no bootable devices found, #2

I’ve got a refurbished Dell xps 13 9350 and for as long as I’ve had it I’ve had issues with it, but I got it for less money so I guess that’s what get for being cheap. My warranty is voided. This message showed up when I tried to turn it on: ‘No Bootable Devices Found’ in the Dell SupportAssist thing. REALLY LONG EXPLANATION: I just had my laptop fixed because Windows 10 just wouldn’t boot up, and this different issue happened on the same day I picked it up from the shop (yesterday). I got home and tried it to see if it was fixed, it appeared to work as it was supposed to, but it was a bit laggy when typing and windows gestures. I checked if I needed to update anything, and it appeared that the latest Windows 10 version was available, so I installed it. After it was done, my laptop was working just fine, no lag this time. The only thing that I noticed was a red X sign next to the little speaker icon at the bottom right corner, so I read about it on the web to try and fix it later. I then closed the lid. When I came back, I opened the laptop to find out that it had a similar problem as before I had it fixed; the Dell logo showed up, and the Windows logo came after, but at the bottom it said something about ‘diagnosing’ and ‘scanning and repairing drive (C:)’. After that, a black screen. I went back and forth with it, turning it on and off, and thought it wouldn’t work (this was my previous issue) but eventually, it did work. The screen saver showed up but my screen was frozen after moving the cursor (also same issue I had before), but after another try, the computer went back to work just fine with the exception of the speaker thing I mentioned. I tried to fix the speaker issue but nothing appeared to work, and I then decided to make a restore point before I shut the thing down. I had not a single restore point, and when I tried to make one, in the Protection Setting section, it said that my local drive had protection turned off. I then went to turn it on, and on the bar that said ‘Max usage’ the indicator was set to 0. I didn’t move this setting because I didn’t know what it did and I specify this because I think it might have to do with my issue. After this, I made my restore point and went back to fix the sound problem. The troubleshooter for the sound issue said that I needed to restart my computer in order to fix itself, so I restarted it twice with no success. I then shut the thing down because it was late, and today’s morning I turned it on to see the Support Assist run a system scan to detect any hardware problems. After that, it said ‘Hardware scan complete with no issues’ and ‘No bootable devices were found... could be a corrupt OS or a boot device not enabled in BIOS setup’ I turned it off and on again, and now the message was shorter, just saying what the title says. Sorry for the very long post, but I figured I’d be better off specifying what happened before in order to get a better answer. I’m also no expert in this field, so if there’s a solution I’d be thankful if you put down exactly the steps I’d need to follow. I’m also not a native English speaker, so I apologize if I made any grammar mistake or let information unclear.