Hey there @BABAL! Welcome to the community! There’s a number of things that could cause this, the most common is normals being non-conforming. Unreal by default culls backfaces, ie you will never see the back of a polygon. This can make parts or all of meshes with reversed normals invisible. Here I have a tutorial that can help you identify where that is and how to fix them (in blender):
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If that’s not the case, it could be your blender export settings or your unreal import settings. Double check the FBX was exported properly as well. I can’t count the number of times in Blender I forgot to check all of an object and not just selected meshes at 4am at night. If it’s all meant to be one piece, verify it is all joined before bringing it into unreal, as otherwise unreal will unpack it into multiple pieces and break it’s form up.