First it is best to understand what tree roses truly are. Please take a look at the article at this link to learn more about them: https://www.gardeningknowhow.com/ornamental/flowers/roses/learn-more-about-tree-roses.htm
Then go to this link about why roses sometimes lose their leaves: https://www.gardeningknowhow.com/ornamental/flowers/roses/leaves-falling-off-roses.htm
The rosebush could be root bound if it is in a container or pot. If it is still in the pot it came in I would recommend transplanting it into a new and bigger pot with drainage holes in the bottom. Give it some water with both root stimulator and two tablespoons of a product called Super Thrive in it (two gallon watering can). Water it with this mixture two to three times at 10 day intervals. It should start to perk up. Then you can give it a bit of rose food mixed well into the soils and watered in well. I recommend using an organic based rose food that not only feeds the tree rose but also helps build up its soils home. If the weather has been extremely hot for several days in a row it could be in heat stress. In that case still use the watering mix I mentioned above but also during the cooler hours of the morning or evening, wash the tree rose down with a watering wand. Rinsing the foliage well can go a long way towards relieving heat stress. Just be sure to do it during the cooler morning hours or cooler evening hours.
could be it's in shock, feed it with rose food, and water it , prune any brown or dead limbs off .. it will let you know if its happy where you planted it..