Generally, when you find that you have a jar that failed to seal, you move it to the fridge, and use it up quickly.
... but by now, you've had the jar at room temperature for a really long time, especially if you didn't move it to the fridge immediately after you asked. If it had been a few hours after canning, especially with a high-acid food like tomato sauce, I'd have no problems recommending that route.
As it is now, I'd recommend composting it and recovering the jar for your next batch. You can decide for yourself how much you're willing to risk getting sick over wasting the food (how hearty your immune system is, how close you are to scraping by 'til the next pay check, etc), but if I were going to eat it, I'd bring it back up to boiling for a while (with the lid on, or a splatter screen at the very least) and only eat it myself, not serve it to others.