Q.weeping cherry tree
The long flowering stems/branches and leaves on my weeping cherry tree have died but new growth is appearing at the base of the tree. It has been extremely hot with very dry weather and I have not faithfully watered my tree. I am thinking this may be the problem. What is your thought? Do you think it could come back or do I need to cut off all dead long limbs? Thanks in advance.

It sounds as if your tree has died and that the new growth is coming from the root stock.
Check the branches by bending them. If they snap or break off they are dead.
You can also to the bark scratch test to see if the tree is alive .
This link will help you with this.
https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-scratch-test.htm
Here is a link with care information.
https://www.gardeningknowhow.com/ornamental/trees/weeping-cherry/weeping-cherry-growing-tips.htm
If you have little to no viable live plant material on above the graft line the tree will not recover.
This article has information on watering newly planted trees.
https://www.gardeningknowhow.com/ornamental/trees/tgen/watering-newly-planted-tree.htm