In MO open the 'Saves' tab in the right hand pane and "mouse over" the save that you are trying to use. There will be a popup with a screenshot of the save and above that a list of missing plugins that this save expects to have installed. If the list is small, perhaps just one or two, I would try re-installing them and then try the save. If the list is more extensive I would bite-the-bullet and try a new character.
Of course if the list is extensive, and you can figure out which of the mods would make changes to the character's head/helmet, you could also try installing that mod.
It is always a good practice to not make changes to modlists mid-game if you want to keep playing with that character. Even with mods that shouldn't affect saves there is always something residual. Maybe it's benign, maybe it removes your head.