No Boot Device Found.

Dell Inspirion 13-7353

System BIOS: 1.14.1

Service tag: 

ePSA: Build 4304.09 UEFI ROM

I updated Window 10 yesterday, and woke up this morning to finde out that my laptop went crazy.

When I turned it on, the supportassist popped up and said, "Operating System Loader has no signature. Incompatible with SecureBoot. All bootable devices failed Secure Boot verification."

First, I went Set up(F2), then Boot Sequence to change boot into Legacy.

It didn't work. Now it says "No Boot Device Found. Press any key to reboot," and keeps on going on loop.

Secondly, I disabled the Secure Boot, but it's  still stuck on "November Boot device found."

Thirdly, I tried hard reboot, opened the back and unplugged/replugged the battery, HDD, and CMOS battery. Nothing worked.

Lastly, I read online that I should remove HDD off the laptop and if it passes the Dell Logo splash screen, it means HDD is faulty. I tried it and it went past the Dell Logo. So does it really mean I have a faulty HDD?

I don't know much about hardwares and stuff so it would be very kind of you guys to help me☺