Half way through ride today my left hand shifter suddenly decided it wouldn't shift from middle to big ring – no crash etc to knock it out.

Have played with limit screws as front mech not moving far enough but hasn't made a difference. Is the shifter broken?

check somehing isnt jamming the mech like a smll stone or something, if not undo the cable and see if the mech will move by hand, if i do's then its probably the shifter

Could be lots of things. Check your cranks. I had the same symptom last week; a few miles later my left crank fell off. 😕

SRAM X9?? if so it's a known warranty issue…

i use sramx9 m8 and ive had my front mech for a while now, i used to have a bash ring on but now have a big ring, mine dosent shift into it unless im in a high gear, i dont know if this is how its supposed to be as its a long time since i used a big ring, or like you say, try messing with the limit screws or buy a new mech if you have tried everything else.

Cable stretched or slipped a little.

can you push the mech by hand into the correct position?

is it a plastic bodied shifter?? I found that the plastic had cracked on mine once and it was flexing the shifter therefore not pulling fully to get it on the big ring…

also, give it a reet good clean and gt85 ?

could be caused by many things…but almost certainly not limit screws

+1 for a small stone or crud in the mech.