XPS 8900, no bootable devices found

possible hdd died or OS on hdd is corrupted.  can you enter bios to see if hdd is still detected?  if hdd still detected by bios try a fresh clean install of OS on it.

@redxps630 is spot on. It could be a failed drive or corrupt Windows installation. You can also try running diagnostics (tap F12 immediately at boot) and watch for any reported drive errors.

@Leavey  We can't see your images.

When was last time you replaced the motherboard battery? You may want to start there, if nothing else but to rule the battery out for ~$2. Check the Service Manual for details.

1. Power off, unplug
2. Press/hold power button for ~15 sec
3. Open case and remove motherboard battery
4. Press/hold power button for ~30 sec
5. Install fresh CR2032 3-volt coin cell battery
6. Close up and connect mouse, monitor and keyboard
7. Reboot

If that doesn't help, reboot and start tapping F12. When the menu opens, selection option to run the Diagnostics, especially the full boot drive tests. Copy error message(s), if any...

Thanks a lot for the tips. The diagnostics tool in F12 (it took 6-7 hours to run this) didn't show any problems for the hdd, then I tried re-installing Windows 10 with a USB but his was not possible because the only possible drive to install Windows was not accessible "MSB reserved". I didn't try to replace the motherboard battery yet.

Re: the only possible drive to install Windows was not accessible "MSR reserved"

assuming you are using USB installation media prepared by MS Win 10 media creation tool,

you should consider do a clean install of Windows which means choose custom option, delete all old partitions on the hdd (you will lose all data), then create new partition (when you see only drive 0 unallocated space), format, proceed to install.  The MSB reserved partition currently on your hdd is one of the old system partition.

Can't hurt to replace the battery before going any further.

Did you try doing a Startup Repair after booting from the Windows USB stick, rather than a clean install? That might save you from losing your files and having to reinstall everything again.

Thanks a lot, guys. Trying out all the suggestions above, so far no success. I have a c:\ which is an ssd which contains my old OS, but I can't run the Windows re-install or repair on it. When I try to install Windows on my d:\ which is a normal hdd, it says that you cannot install Windows on it. Still haven't tried to replace the battery though.  

Re: I have a c:\ which is an ssd which contains my old OS, but I can't run the Windows re-install or repair on it.  When I try to install Windows on my d:\ which is a normal hdd, it says that you cannot install Windows on it.

change BIOS>SATA operation in bios from RAID (default) to AHCI. I would remove the hdd drive (current D) from pc then try attempting clean install of Windows 10 on ssd (current C)

while your pics are pending Dell approval, you may want to try to boot from USB installation media and then try a clean install (follow the pic guide) on the ssd, rather than re-install or repair of existing Windows 10, which might be causing your error.  

Problem solved. I brought my desktop to a PC repair shop. It turned out that my ssd had died, so they replaced it with a new WD 1TB M.2 ($ 90). I re-installed Windows 11 and now it works again. Unfortunately I lost all the data on my c:\ drive.

If you got the old SSD back, you can try installing it in a compatible external (SATA) enclosure as a secondary (non-bootable) drive and see if you can recover your files...

Thanks, I will try and post it here if it works.

I would pull th power cord, then press and hold the power button for ten seconds, replace the power cord and try to turn on normally.

also, i find that when certain mobile phones are plugged into a USB port the problem you describe appears... remove the USB device and try booting again from power off.

could be true, could be they just sold you a new drive as that is the quickest solution. if it happens again consider what i suggested above.