I had installed Windows 10 successfully. While messing around with my other Linux partitions, a partition delete went awry and on booting I was dropped into a GRUB rescue prompt. What I wanted was the Master Boot Record to be restored to boot into Windows 10.
On another computer, I downloaded a Windows 10 ISO and created a bootable Windows recovery USB disk as described here.
I booted with this USB disk, chose Troubleshoot -> Advanced Options -> Command Prompt.
At the command prompt, type these commands:
$ bootrec /rebuildbcd $ bootrec /fixmbr $ bootrec /fixboot $ exit
After restarting I was able to boot into Windows 10.