Inaccessible boot device and PC

There could be many reasons for that but it is generally seen that after the updates sometimes such a thing happens in Windows 10. So the prime reason for this is due to the restriction of access of Windows to the system partition during the startup process.

Some of the things which can be tried are -

(1) If the recent updates have caused this problem then removing them one by one may help in the matter. If you are conversant with DOS then you can do it by going in command mode and dismantling the recent packages. Otherwise take help of a professional.

The steps are -
Settings > Update and Security > Recovery > Advanced Startup > Restart Now.

Next is -
Troubleshoot > Advanced Options > Command Prompt.

Now in command prompt go ahead as follows -
dir c: (I am assuming that Windows is installed in the C drive)
Run Dism /Image:c:\ /Get-Packages.
Identify the most recent and make a note of it.
To remove it -
dism.exe /image:c:\ /remove-package /[package name]
After this Reboot your PC.
Remove the next recent if not successful.

(2) Sometimes old drivers create this problem. You can try it by updating them.
The procedure is -
Open Device Manager.
Expand the IDE ATA/SATA controller sub-menu.
Right-click on Standard SATA AHCI Controller.
Select Update Driver.

(3) Sometimes enabling AHCI mode in your system BIOS also solves the inaccessible boot drive error.

For this the steps are -
Enter the BIOS during boot ( by pressing Escape, Delete, or one of the Function keys).
Advanced Options menu.
Set AHCI Mode.
Change the value to Enabled.

(4) There is one more option but you will lose all your data in the PC.

That is to invoke 'Reset Windows 10 back to a clean copy'.

Try the above if you have basic exposures to computer systems and softwares otherwise go to a qualified software professional.