What happens if you have a kidney stone but you never pass it?

That depends on where it is. If is sits out of the way in the kidney it does little harm. If it's location blocks urine outflow at any point, the backpressure created can injure tissues. If it is large enough to block the outflow tubes of the kidneys it can cause some of the worst pain ever & trigger other damage.You can help yourself by providing details specific to your case for a focused answer

Agree with Dr. Ferguson. Even if the stone does not cause blockage to the flow of urine, it is likely to promote urinary infection.