Happened on Jacobs Ladder today. Lost all shifting. I assumed it was mech hanger, but it looks like the little screw you tighten to allow a larger range cassette has sheared off, would this cause these symptoms?
It’s a new bike to me, Whyte T129s and SRAM X9 mech, so I’m not so familiar with the parts, here’s some pics….

I’ve just found a screw that’s the same as the snapped one, now chain goes onto the 42T ring but won’t shift at all?

Not that familliar with SRAM but that is called the B-limit screw on shimano stuff. It controls the distance between the top jockey wheel and casette so if it were to fail then yes it probably would impact shifting as the mech might hit the cassette.

If you are on good terms with your LBS then they’ll probably give you one. If not ask a mate or have a look in your box of miscellaneous screws for one that fits. Nothing particularly special about it.

Does the mech move freely, loosen off the cable and then see if that moves as you pull the shifters – might be snapped or snagged.

The screw might have been broken forever.

Try the park tools site for a guide, loosen the cable off then set up from scratch as instructed altering the high and low stops.

would this cause these symptoms?

It looks like as well as the screw broken you are missing bit of the mech hanger in the frame?
The mech also has a couple of bash marks on the edge of it – you may have walloped a stone or two of the mech, causing the breakage.
*maybe*

Screw I’ve found fits off an old XT mech I had lying about, just stuck on the 42T cog now and won’t shift down.

Update, undid the cable and it appears the shifter isn’t shifting at all.