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