I last rode the bike on Sunday. Throughout the ride I noticed the front shifter (grip friction shift, Shimano) was feeling "looser" than normal (hard to describe, it didn't feel "right" and seemed to be clicking louder than usual). However nothing seemed out of order and shifting was functioning fine.
Today, haven't not rode the bike since, I went back out on the road with it in 1 (smallest sprocket, left there from Sunday) and immediately found it will not shift at all. After pedalling wildly at a wonderful 8mph for a while to a safe stopping point I found the cable completely detensioned and hanging beneath the frame. Twisting the shifter to "H" (limit of its travel) tensions the cable enough that it is nearly in the correct usual position for "L" when the bike is working. It's impossible to produce derailleur movement.
What could have caused this and what do I do now? Not experienced at bike repairs; last time I tried a front derailleur repair, it went horribly wrong, but I want to know what the fault is and whether it's easily doable.
To the best of my knowledge, the bike has been touched by nobody between Sunday and today. The derailleur seems to be working fine (manually pulling it).
The suspicious shifter behaviour on Sunday makes me think the shifter may be at fault, but I do not understand how the cable has suddenly completely slackened. I don't want to touch anything lest I make it all worse - part of me wants to just retension it with the pinch bolt, but I suspect this may not solve the problem.
Possibly related: I'm aware the H-stop on the front derailleur is improperly adjusted, and I've had the chain fly off the outer sprocket on shifting a few times lately. I really need to get around to adjusting that, but suspect it's unrelated to the sudden cable tensioning problem.