In the future, should that happen in Silo, select the affected faces (if the all the faces are part of its own material, you can select by material) and in Silo, just go Modify>Reverse normals  OR Select the object as a whole and choose Modify>Unify Normals

 

Silo is still my favorite modeler (despite owning Modo and Max) and I hope Nevercenter gets back to it soon.

 

EdiT: You can also check the normals before export by going to Display>Object Display Mode> Show Normals