It comes . . and it goes . . . it comes and it . . goes. Seen this tidal flow over ten versions of VP. One option is to track down the Help file and copy it to desktop.
As Grazie proposed I made a shortcut to the vegas170.chm -file on the desktop. Clicked it and Help starts as expected - but when I start typing something into the index-field it disappears from the desktop (crashes).
Solution (Or rather a work-around):
1) Start HELP from a shortcut or from HELP in Vegas 17
2) Click Search and type something + Return
3) Now click Index - and now it does not crash any more.
Seems to be a lasting solution - even if I restart Vegas 17 and use Help/Index it seems not to crash any more 🙂
Edit1: Closed V17 and rebooted the machine - and Help/Index still does not crash. Quite a pesky error - and difficult to find when testing.