We have all had the opportunity to round off a frozen bolt or two. A socket might split, or we might have wished we had used a 6-point socket instead of a 12-point for just a little more surface contact. That is just before the bolt rounded off a few corners. If you have been using a 12-point socket, switch to a 6-point. That extra contact might save the day. If not, and you are working with US. size bolts, go down to the very next size slightly smaller in a metric. Example from a 9/16″ to a 13mm. Gently tap it onto the bolt. If you have rounded it off enough, it may fit, giving you a fresh start. If not the next step may be the heat wrench.
.