Front derailleur cable suddenly very slack (no shifting), not sure what's caused it 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.

ilmiont
Most likely cause is that the front derailleur cable simply slipped through the pinch bolt. The derailleur cage has to put significant force on the chain when shifting to the large chainring which can overcome the clamping force on the cable.

Another cause of shift cable becoming slack is the housing popping out of a housing cable stop on the frame. As you describe the shifting feeling progressively looser my bet is on the cable slipping. The cable could also have snapped but that tends to happen when shifting and is sudden and obvious.

Couple of things to check: Hold the cable taught and verify that the shifter is pulling and releasing it. Manipulate the derailleur by hand a verify that it will swing through its proper range. 

Re-secure the cable and re-adjust the derailleur and you should be OK. It's a good idea to run thorough a setup procedure as front derailleur adjustment can be tricky. Park Tool has a good web page and video with step-step instructions.

https://www.parktool.com/blog/repair-help/front-derailleur-adjustment