Can I reprocess jars of salsa that didn't seal
Per Joe, the answer about jam jars that didn't seal applies to you as well.

Given that the salsa has now sat out all night unrefrigerated, it would not be safe to re-do the canning process.  If you had checked them immediately, you could probably have used new lids, cleaned the rims of the jars, and hot-water-bathed them again.  But not after 8-12 hours at room temperature.  If they have very high acidity (PH less than 4.0) they might be fine, but why take a chance?

Instead, I suggest putting those jars in the fridge and eating them yourself within the next few days, checking them for bubbles that indicate that the salsa is going off.