No bootable device found. Boot device not found. No boot device available. These are slightly different ways to say the same thing by various manufacturers; when it happens, the computer has gone through its startup process, looked for a "boot" disk or partition to start from and couldn't find anything it recognised as such. The next 10 minutes determine if you're going to have to spend 2 minutes adjusting some settings or if you'll be needing a recovery operation.
The one piece of information that is most helpful regarding this error is which entity is making the statement. This is not Windows, macOS or Linux: they haven't even begun yet. This is an error generated by the firmware, either by the BIOS or UEFI, prior to an OS being loaded. When the firmware completes its self test, it goes through its list of boot devices in search of something with a bootloader on it. Once it reaches the end of the list, it gives up.
This timing allows you to pinpoint the location of the problem: somewhere between the firmware and the drive. The drive may exist, but the firmware cannot read it, or the drive simply does not respond. These two possible locations represent entirely different scenarios (one leads to a menu selection while the other leads to a recovery quote), although the message itself doesn't indicate which. Fortunately, a one-minute review in the BIOS will.
Press restart and hold the setup key during power-up: usually F2, Del, F10, or Esc, depending on the maker. Search for a page listing your storage devices (usually labelled Boot, Storage, or the main information screen) and then determine whether your drive appears anywhere on that page.
If your drive shows up: the hardware is working, and there's likely no serious problem here (the benign branch). Common causes of problems include a boot order that has been reordered due to a USB stick left in a port, a firmware update that changed the Legacy/UEFI or Secure Boot setting, or a corrupted boot record as a result of an abrupt shut-down. Set the correct drive to first in the boot order and retry before deciding there is anything seriously wrong.
If your drive does not show up: stop. This means the firmware is telling you that the drive is unable to communicate on its own connection: a dead/failing drive, a dead/failing controller board, a disconnected internal cable in a dropped laptop, or the sudden-death pattern that SSDs are known for. No matter how much repair you do with Windows, it cannot fix a drive the computer itself cannot see. Every subsequent boot attempt puts additional load on the potentially limited remaining life of what could be a drive that is already going bad.
Advice online about troubleshooting this error typically includes a series of commands that are written to a possibly dying drive. Automatic Repair loops, bootrec /fixmbr, /fixboot and /rebuildbcd are examples of commands that rewrite boot structure: completely harmless on a healthy disk whose boot record was simply scrambled; however, they are extremely destructive to disks that have failing sectors. With each command to force write data onto a disk with failing sectors, you run the risk of overwriting data that you need while pushing weak areas beyond their ability to read.
The reinstall is the most costly. The "just do a fresh install of Windows" method will end with you having an empty file system over all of your pictures. Running chkdsk on a failing disk (the other commonly suggested solution) does hours of high intensity reading & writing across the same problem area(s) as before. If the disk is invisible to the BIOS, or makes noise (clicks, buzzing, spinning up and down) or gets hot while doing nothing, then the only true answer is: turn it off and keep it turned off.
Work done on the branch for drives invisible to BIOS is straightforward and occurs entirely at hardware level and not within the operating system. That is to say, a sector-by-sector image of the entire disk is made using a piece of equipment that never attempts to write anything back to it, the file system is rebuilt based upon that image, and your files emerge from the copy. An SSD that went missing from BIOS overnight typically has either a faulty controller or faulty firmware; a hard drive that became invisible after you knocked it or after it gave a "click" has a mechanical issue. Both are routine, but neither is repairable by trying to boot it again and hoping something magically works.
Before anything leaves: it's the drive we want, not the PC. Take it from the laptop or tower. Any local PC shop can do it in just minutes if you'd like. Once you get a quote, then send the drive itself. The diagnostic is free. A fixed £300 + VAT applies to all single drives in the standard band. In most cases, nothing will be charged unless your data is returned. If the BIOS can see the disk, then you probably need to tweak your boot order and get a coffee... And we will gladly say so for free.
Each time you restart, you're causing this potentially worn-out disk to spin up again. Each time you "repair" you're writing to the only copy of your data that exists. Turn off power, remove the drive and let the free diagnostic tell you what is actually wrong with it. We'll write an agreement for a fixed cost of £300 + VAT if recovery is required.