After a power failure my Win 10 Pro (an free upgrade from Win 7) would not start up, only showing a flashing cursor on boot., Attempts to 'force' it to enter Automatic Repair Mode initially failed, but after two days of trying suddenly entered 'Diagnostic' mode - where it stayed for 24 hours! Further restarts gave me either Repair or Diagnostic modes, but no further progress.
Ordered a Windows 10 Recovery Disc as a last resort, although with little expectations.
How wrong I was. Once I booted up from the DVD (with the correct settings), went straight to the Repair Screen, and within an hour I was up and running again. Although a further glitch happened the next day, a further repair has sorted things out and now I have a stable, usable system.
Thank you Software Recovery World.