My problem: Gigastudio launches as usual, and I can load gig files as normal. But it no longer responds to MIDI, nor can I play the little on-screen keyboard on the Midi Mixer page. Nor, finally, can I any longer assign a gig file to a midi channel by selecting the channel and then clicking on a loaded file.
I don't believe it's a Midi-related problem because Giga on my other PC, receiving Midi from the same source (SONAR), is working normally
Following a tip I saw here, I went the Control Panel/Administrative Tools/Event Viewer route and found a Giga-related error message dated March 30th, which is when GS stopped working.
Well, I fixed it, thanks to this forum, so thank you everyone. A couple of hours sifting through relevant threads delivered the solution - a complete and thorough uninstall, following the instructions posted here, before re-installing. I had already tried un- and re-installing a couple of times, but only using Windows' own utility, which obviously isn't very exhaustive.