Pruning clematis will never hurt them and the worst it will do is postpone your blooming. Jackmanii is a type III clematis so pruning it now will not hurt it at all since it blooms on new wood. Not sure where you are in the country and what your weather is like now or when your last average frost day is but if you are still likely to get frosts, I would hold off pruning now until after your last average frost date. Pruning stimulates growth and bud break so if you are likely to have more cold weather, hold off pruning now until after that time. Pruning a type III clematis later in the spring is not going to hurt it. Your flowers will just appear later in the season than they would have if you had pruned it early. Prune your Jackmanii down to 4 to 5 inches and let it come back out.

Thank you so much! I am in zone 6 (NJ), so I will hold off pruning for a few more weeks. I'm just so happy that it's starting to grow again.

You are welcome Loveflowers. Show us pictures when it is blooming.