Shifter stopped ratcheting/shifting

I had a similar problem earlier this year; sluggish or non-existent shifting from the lever, but could change by pulling directly on the cable on the downtube.

The original grease in the shifter had solidified (due to age and low ambient temperatures), so I sprayed a well-known oil-based solvent liberally into the works while moving the lever. Eventually this cleared the gunk and allowed proper functioning; I then sprayed a PTFE lubricant into the mechanism. It's all worked perfectly since then! (A little extra PTFE lube will be added before winter - I don't think it'll be needed before then). 

Relative to the cost of cassettes and chains I have always found the cost of inners and outers to be low enough to change them regularly, say every second strip down service. Consequently, I've rarely had a problem shifting but if I've noticed some a slight problem I've taken the inner out, cleaned and waxed it and put it back in. A squirt of WD40 into a shifter helps as well.

Changing the inner is never a bad thing to do and as a first thing to do it can sort out a lot of shifting problems.  Inspect the outer, and especially if it's short cable runs, changing the outer near the rear derailleur is easy and a thing to do whenever changing the inner.  It's the place that wears the most, because of the tight turn and the closeness to any filth.

Luckily you tend to get far more outer than you ever need with a cable, so getting an inner on it's own and using the scraps left over works.  Especially if you add some Middleburn Cable Oilers, which means that very small peices are required, and you lengthen the life of your cables and can easily lubricate them.  Win Win.

Your shifter seems quite recent so it's probably not the lubrication in the mechanism.  But being someone who likes to use almost vintage parts to keep my early 90s Getting to Work bike going, Shimano grease turns into some claggy nightmare over the long term, even on some New Old Stock items.  An good clean with a degreaser, some dry degreaser and then using an almost pure PTFE product gives everything a lot more use.  I like Finish Lines Pedal and Cleat spray for this.  It's great for the very long term care for my brake lever pivots too, I have worn far too many of these irriplacable components over the years and getting longer life out of them is important.

Change the cable first, if the end breaks off in the shifter it is close to impossible to remove.

I'm with Rich_cb, but rather than just checking, I'd go for replacing the inner cable to start with as they do get worn and damaged. Whilst replacing the inner, you might find that the outer needs replacing too (the outers tend to last for ages unless they've got damaged).

Worth checking the cables and outers.

Had a similar problem with my rear derailleur, turned out it was a frayed cable sticking in the outer.