I’ve had this happen to me in the past and just came to a halt and got sick of what is causing this to happen? Am i pressing something wrong? am i modeling it wrong? Is there a way for it to not show up like this? If you could help that’d be awesome 
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Are you making the hat 0 thick, a plane and not a cube for each of the faces?
Its a circle starts out as if it is a plane
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It looks to me that your normals are inverted. Try flipping them.
How to do this in blender : How do I flip normals on this model? - Blender Stack Exchange
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Your normals are inverted; in short, ROBLOX can only show one side of a face, if you look at it backwards you’ll see nothing.
You can usually tell the way normals are facing in the viewport because you’ll have a slightly darker shading if the normal is facing away from you. If you enable Backface Culling, the viewport will display normals from a single side like it does in ROBLOX.
To enable Backface Culling, it is in the sidebar under shading (open sidebar with N).
It can be good for debugging, but I wouldn’t recommend having it enabled 24/7.
At a minimum, to correct the normal facing, you’ll need to select all the faces in Edit Mode and then use the shortcut CTRL N or search for the function Make Normals Consistent.
This will make your model at least look normal from the outside, but you’ll still get the same error with
invisible faces, but from the opposite direction.
To finish it out, apply the Solidify modifier to turn your flat geometry into a solid model; you should be fine, but again check to verify your normals are all facing outwards. In the settings you can change which side it extrudes from and by how much, so play with that until it’s where you want it.
In addition to CTRL N, the use of CTRL SHIFT N will directly invert a normal if you need to spotfix something that Blender is consistently making backwards.
Welcome to 3D modeling, you’ve experienced one of the common pitfalls everyone experiences early in their career; keep at it and you’ll master it in no time. 
An additional comment:
Perhaps you are intending for the ‘low-poly’ style, but if you are wanting a smooth helmet, something to consider is applying Smooth Shading (left panel under tools) and an Edge Split modifier (same place as the Solidify modifier).
This can be used on most models without harm and will make stuff that should look smooth be smooth, while retaining your sharp edges. Blender modifiers are super powerful and definitely worth looking into to do many interesting thing in a fraction of the effort.
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In blender go intro edit mode, then select all the vertices on the mesh then press control-shift-n all at once
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If you make a model in Blender or Cinema4D and import it into studio sometimes it doesn’t appear, it’s because of the thickness, if you choose a ‘plane’ then it’s not going to show up because it’s super thin, try modelling with a cube and make it thinner if you wish but don’t go for 0 thickness, hope this helped.
In blender 2.8 you can view normals as red and blue and then flip them accordingly.
First your normals are inverted, and as other users mentioned you need to fix them, however if you look from inside the helmet it will also be invisible, the only difference you make when you invert normals is that you change the direction of the “invisibility”, if you want to make the hat more 3D without much work, use the “Solidify” modifier, just an extra tip.
Apply a modifier called “Solidify” and it will be fixed.
To apply it select the model > Goto modifier menu > Click solidify > apply and then export it from blender and import it in studio
easy peasy 
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Check the date before bumping a 12 month old thread, have you tried the provided information above and see if it worked before attempting to reply? If you haven’t tried the methods above please avoid bumping old threads that already been answered and solved.
If your model has missing parts it’s because your normals are inverted. The face’s normals are inverted inside out. This could be fixed by using the shortcut (CTRL + N.) You’ll need to flip your normals in edit mode select the mesh and flip it. If your wanting a different approach try looking in the upper-left corner of the 3D view menu and click. Mesh > Normals > Flip Normals.
Try researching the problem your having first before attempting to reply to something that’s old.
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uh yeah this was when i was first starting out in blender idk why someone bumped this??