my 3d model is seperating to pieces on xna :( .fbx problem

Please , help me, i have a big problem :( . i created 3d object in 3ds max , when i rendering it , i can see original model, but when i use it on xna , my model is seperating to pieces, all of the object has different location :( , What is the problem and how can i solve this problem :( . i think i have to merge model on 3ds max but i don't know what to do. (i export .fbk file and i use it on visual studio.)