The answer to 'sink won't flush' is that the most common causes are a clogged drain, a broken or malfunctioning plumbing mechanism, or a problem with the sink's water supply. Symptoms may include slow draining, gurgling sounds, or the sink refusing to fully empty.
Some DIY solutions to try include using a plunger to dislodge the clog, using a drain snake or auger to clear the pipe, or checking for any debris or buildup in the drain. However, if the problem persists or you're unsure of the cause, it's best to call a professional plumber who can properly diagnose and fix the issue.