I've had the black screen and white cursor before on two occasions and the only thing that would boot up was a Kaspersky Rescue Disk and after its scan I was able to boot normally, although they weren't the result of running Windows Repair.
https://support.kaspersky.co.uk/viruses/rescuediskIt has been reported that the repair program isn't actually creating a restore point but it does back up the registry.
Unfortunately you need to be able to open the program to restore with that.
If you have access to another machine you can create Win 10 install media to see if the machine will boot from that.
You can create the install media at
https://www.microsoft.com/en-gb/software-download/windows10 but read the instructions first as the procedure differs depending upon if you want to use a DVD or USB.
To change the boot order, press and hold F2 as you switch on then use the cursor keys to navigate to the Boot tab and then again to select the media you want to use and use F5 or F6 to move that to the top.
However, if using an UEFI BIOS you will need to disable Secure Boot to boot from external media.
Insert the install media - press F10 to Save and Exit - press enter to confirm.
If using a DVD then press any key when prompted to boot from CD/DVD.
You may be presented with an inverse window with Windows Setup (EMS Enabled) highlighted - just press enter.
Use the dropdown to change/confirm the Time and currency and navigate to the Install screen and click on Repair your computer.
Click on Troubleshoot and while you could use the Restore option to see if it will work from there, you can select Command Prompt to perform these cmds.
bootrec /fixmbr
bcdedit |find "osdevice" and then using the partition letter instead of the x I've used, enter chkdsk x: /r
For clarity that is a Pipe symbol before find and is the uppercase of \ and you need to use the bcdedit cmd because it doesn't always see the volume as in C: in that mode.
Its report will remain on screen.
Follow that cmd with this one, again using your partition letter instead of the X: I have used.
sfc /scannow /offbootdir=X:\ /offwindir=X:\Windows
Note the space before each /
Enter exit to close the cmd window, remove the install media and use the back arrow to Turn off the PC and see if it will boot normally, but let me know if either the chkdsk or the sfc reports any anomalies that it is unable to repair.
This is a valuable lesson learned that you need to get into the habit of creating regular system images onto external media such as onto an external HDD and then you would have been able to restore with that.