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The shoots at the bottom are coming from the roots that the top is grafted onto.
If the tree limbs are still alive; bend do not snap off, you can give the top more time to recover. Remove the growth at the root line.
Make sure the tree is receiving enough water.
Most reputable growers will guarantee a new planting for 1 year, so if you do lose the tree you should be able to get a replacement.

This is, actually, a common method for trimming these, and will cause no harm. It will grow back fairly quickly, and you will have to do this again by next year. This article will help you prune these in the future: https://www.gardeningknowhow.com/ornamental/trees/weeping-cherry/pruning-weeping-cherry.htm

Well draining soil is key to the successful growing of a Weeping Cherry Tree.
https://www.gardeningknowhow.com/ornamental/trees/weeping-cherry/weeping-cherry-growing-tips.htm

This sounds like it could be Shot Hole Disease.
How severe the disease was and if you were able to successfully treat is the issue.
Loss of all green leaves is not a good sign.
You can wait it out and make a determination in the spring.
I would check on the return or replacement policy at the store you purchased from.
Most reputable garden centers will replace a tree in the first year.
These links will help you.

https://www.gardeningknowhow.com/ornamental/trees/weeping-cherry/weeping-cherry-growing-tips.htm
https://www.gardeningknowhow.com/ornamental/trees/weeping-cherry/pink-snow-showers-weeping-cherry.htm
https://www.gardeningknowhow.com/plant-problems/disease/shot-hole-disease-fungus.htm

This sounds like a case of Hole Shot disease. The tree was likely stressed from it's time at the garden center.
Copper fungicide is recommended treatment with the pruning and removal of infected plant material.
I would return the tree for a replacement.

https://www.gardeningknowhow.com/plant-problems/disease/shot-hole-disease-fungus.htm