Sorry 27hrs on duty, I should have asked if the stones that have fallen into the basement were from directly below the posts or from the area between the posts? The stones directly beneath the posts are handling most of the weight so the loss of one of them is akin to a three alarm fire.
If it's just perimeter stones falling then your repair is less urgent so you can take some time and come up with the best approach. You may be able to effect repairs without lifting the barn at all.
If stones from directly below posts are missing then you want to get the shored up as quickly as possible before the barn sags out of square.
