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There are many similar questions on SE, but none I think are the same.

This morning, commuting on my bike with Shimano Nexus 7, I suddenly realised I can't shift anymore. I have a grip shifter, it feels like it is stuck on 3rd gear. The hub works fine as a single speed, no grinding or other issues, and is stuck at what feels gear 2-3, certainly not 1 or 5+. I can twist the shifter down from 3 to 2, and most of the way to 1 with no unusual resistance, but there is no change in gearing. Trying to push it to 4th gear is met with hard resistance, as if it were at the end of the range. Cycling conditions this morning are cold but dry in case it matters.

I tried looking at the hub, turning the shifter doesn't seem to move any parts. I tried wiggling the lever where the shifter cable meets the hub: I can move the part used when taking the wheel off the bike, but I couldn't move the actual shift lever by hand (not sure if I should be able to, also was cold / had gloves on). I tried looking at least for changes in cable tension when twisting the shifter - again, not sure if I'd expect to see anything, but I couldn't - would that mean that the cable itself isn't moving?

Prior to this, the hub was 100%, occasional gear slips resolved with barrel adjustment. On this morning's cycle, I do remember a dodgy gear change, but nothing crazy - I eased off the pedals, moved the shifter, resumed cycling, then the actual gear shift happened a few strokes in. Not sure if that's related.

The bike is generally not abused, though also I haven't done any maintenance at all on the hub for years.

Any suggestions where to start debugging this welcome.

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    Does your bike have an exposed area of inner wire for the gearbox? Try pulling sideways on the inner and see if the gearbox actuates. This will help isolate if the issue is in the shifter, or below where you're pulling. If you have a way to get the bike off the ground, carefully do the same down at the right-hand side of the gearbox, where the wire goes into the "cassette junction" ie the turning bit. Mind your fingers.
    – Criggie
    Dec 14, 2022 at 19:56

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Answering my own: weirdly, it turned out to be all to do with the cold weather. I have never seen anything like this!

The IGH would work fine in the garage, which is somewhat warmer than outside. All the machinery worked just fine. Then, minutes/seconds after leaving, depending on outside temperature, it would stop shifting. I'm not sure if it got stuck on the same gear each time, and if it did, whether that was related to my cycling route - I tend to be in a particular gear X minutes after leaving home on my fixed commute.

Temperatures rose and bike fixed itself.

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