Q.Can magnolia tree be cut back to almost ground level
My magnolia is about 20 ft. high and 8 ft in diameter. This year it did not blossom and the tree didn’t leaf out. Then I started to get new growth on some of the branches. Can I cut the dead branches off and hope it will fill in again?

I will caution that pruning is something you are going to want to do very carefully and judiciously because Magnolias are not the fastest growers nor do they heal from pruning cuts well. They should really be pruned as little as possible. However, you may remove all dead, diseased and injured branches. You mentioned growth was starting on some of the branches - your magnolia may just be slowly recovering from a hard winter so keep that in mind when you are pruning. It may look dead, but it might still recover. Here is an article that will guide you on pruning your magnolia tree:
https://www.gardeningknowhow.com/ornamental/trees/magnolia/pruning-magnolia-trees.htm
Here is an another article I would highly recommend which covers the reasons why magnolias do not bloom:
https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-blooming-problems.htm