Have you been disassembling armor at the blacksmith table while wearing the helmet?
There is a bug going around with disassembling armor, which has a chance to automatically disassemble your equipped helm. Which might be the reason why your helm is gone.
If this is the case, then i'm afraid you lost your helm for good.
Happened to me yesterday -- hit R to deconstruct with no item slotted resulted in it eating my helmet. I knew what happened immediately. At least it wasn't something special. I just made another.