Why isn't my sprint key working?

You are probably low on hunger. Eat then try double tapping the W key and holding it after.

You need to tap or hold the sprint button while holding a direction, releasing all direction keys will stop even if you continue to hold the sprint key. Simply pressing or holding the sprint button on its own will have no effect on your game.

A common layout is to use the Shift or Ctrl key as sprint (depending on your crouch key), so you can hold it while using the WASD keys to move.

If this is what you're doing, make sure you're saving your settings after modifying them. It's possible you're making changes and then discarding them accidentally.