No bootable devices found - Dell Latitude E7450

I have a Dell Latitude E5740, running Windows 10. One day when booting up my computer, I received a black screen that said "No bootable devices found, Press F1 key to retry boot," etc. I don't believe that I had installed any recent updates...this seems to have come somewhat out of the blue.

Here are the things I have tried to resolve this:

1. I ran the Dell onboard diagnostics tool accessible from the one-time boot menu. Everything came back as working well. I do not think that my hard drive has physical damage (and there's no reason to suspect that it would, either).

2. I then went into the BIOS and changed the boot mode from UEFI to Legacy, and entered the Dell Recovery & Restore application. When I tried to run the "Smart Repair" option, I received the message "We are unable to repair your computer." Above this message, there is a line that says "Partition Table" and it has a red X beside it.

3. I created a bootable Windows 10 installation USB. I tried running all of the various "repair your PC" options available, and they all returned similar messages ("unable to repair your computer").

4. I also entered the command prompt and entered the following commands to try to make the right partition active (after reading several blog posts on this):

X:\Sources>diskpart

DISKPART>list disk

DISKPART> select disk 0

DISKPART> list partition

DISKPART> selection partition 1

DISKPART>active

DISKPART>exit

X:\Sources>exit

This did not seem to do anything for my problem.

5. I have also checked the BIOS boot order to make sure that the computer isn't trying to boot to something besides my hard disk, unplugged all external devices (except when I was trying to use my bootable USB), etc.

After doing a lot of reading, it seems that the following could be options:

1. Try to run chkdsk from the command prompt to check for (and perhaps repair?) bad sectors
2. Try to update my BIOS
3. Fix MBR of my internal hard disk
4. Format my hard disk

However, I worry that since I would be doing any of these things based only on the advice of blog posts, I could mess my hard drive up even more, making more unlikely that I'll be able to recover my hard drive and get it working again. (I think the first option, chkdsk, is probably safe, but the other options seem increasingly invasive and potentially disastrous if I do them wrong. Also formatting my hard disk would result in losing my data, no?)

Bottom line: I'm pretty sure something is wrong with my hard drive's partitions or partition table. But I just would appreciate advice on what to try next to resolve this problem, or which of the options I listed above would be most effective to try next. Please let me know if there's any other information I can supply that would make diagnosing my problem easier.

Thanks!

Know someone who can answer? Share a link to this question via email, Twitter, or Facebook.