A guitar that's played beautifully for months and then suddenly feels stiff and hard to fret can be genuinely confusing, especially if you haven't changed anything yourself. The good news is that a sudden change in action almost always has a clear cause. Here's where to look.

Humidity is the most common culprit

A sudden jump in action very often points to a change in humidity rather than anything mechanical. When a guitar's top absorbs excess moisture, it tends to swell and dome slightly more than usual, which pushes the action higher. This is especially common around seasonal changes, or after a guitar has spent time in a noticeably different environment to what it's used to.

Temperature swings

Along similar lines, big temperature changes, particularly combined with humidity swings, can cause a neck to shift slightly, changing its relief and, with it, the action. A guitar that's just come inside from a cold car, or that's been sitting near a heater or in direct sun, can genuinely feel different within hours.

The truss rod has moved

Wood responds to its environment, and a neck that's picked up extra moisture can pull slightly more forward curve into itself, increasing relief and, with it, the action. This is closely related to the humidity point above, since it's often the underlying cause of that shift.

Something has actually changed

Occasionally the cause is more direct: a new, heavier set of strings, a change in tuning, or a saddle that's shifted slightly. It's worth thinking back over anything that changed around the time you noticed the difference.

What to do about it

If you suspect humidity, start there, get your guitar into a stable, well-humidified environment and give it a few days to settle before assuming it needs mechanical adjustment. If the action is still noticeably high after that, or if it happened without any humidity change at all, it's worth having the truss rod, nut and saddle properly checked rather than trying to diagnose it further yourself.

The reassuring part

A sudden jump in action is annoying, but it's rarely a sign of serious damage. In the vast majority of cases, it's your guitar responding normally to a change in its environment, and it's a straightforward fix once the cause is identified.