Why can't I pull my model into the Unity scene?

I’ve got a bunch of .mb files in my Assets\Models folder. I imported them into my Unity project, and things worked great. However, I recently made some changes to one of the models, deleted the old one and imported the new edited .mb file into Unity, and first of all, Unity freezes up whenever I import anything. When I restart it, the model is in the folder but it can’t be pulled into the scene. I’ve included a picture to illustrate what it looks like. If I try dragging the model into the scene, the cursor turns into a blocked icon, and nothing happens when I release the button.

Unity also does not preview the model when selected.
Import Settings are exactly the same for all the different models.

first of all make sure maya is installed in your computer so that you can use .ma file directly in unity.

secondly ,you need to open your asset folder and copy your required File in it,after that you will start your unity .i am sure you will get your file perfectly because some time when we drag file in unity its corrupted .