Even easier,
With RoboHelp not running, in Windows rename the HHK file
just to have a backup. It doesn't matter whether you move it or
leave it there.
Launch the project in RH, which will generate a new,
initialized HHK file. If you change your mind and want to go back
to the original, rename the newer one and restore the original name
to the original HHK file.
A little trickier, but doable, is merging the two HHK files
with a text editor (RH not running), if that's what you need.
HK