Loose tar-coated stone chips lodged in a depression on the exhaust system under the clutch release arm causing the clutch to stay disengaged. Caused mild panic while trying to find, with useless clutch, a suitable place to put the bike safely on the stand to fix the problem. No tools required, just a short stick to rake out the offending stone chips. Glad it wasn't at night! Could happen to anyone, anywhere. 🙄