The reference ID for Yngol's Helm is 000295f3, and the console commandplayer.additem 000295f3 1should add one "Helm of Yngol" to your inventory. Keep in mind though that this will create a Helm of Yngol, not teleport the original one from wherever it is into your knapsack.
If you took it before and stowed it somewhere you will end up with two of them if you find the first one again. In which case you can useplayer.removeitem 000295f3 1to get rid of the extra helm. Or just keep them both, or sell one, whatever.If you didn't get it when you cleared the barrow, it might never show up again in your game unless you use the console command. If the body it was on was no longer there when you returned, then it probably got cleaned up by the game and any items left in its inventory are gone. Thanks everybody.
I've found in the elder scrolls wiki the description of a bug:
"
PS3 360 There is a known bug which seems to occur if you have accepted the quest to fetch the http://elderscrolls.wikia.com/wiki/Helm_of_Winterhold for the http://elderscrolls.wikia.com/wiki/Jarl of http://elderscrolls.wikia.com/wiki/Winterhold prior to entering http://elderscrolls.wikia.com/wiki/Yngol_Barrow. The bug replaces Yngol's Shade with a Draugr Deathlord or http://elderscrolls.wikia.com/wiki/Dragon_Priest and completely removes the skeleton and the helmet with it. It also causes the door in Yngol's final chamber to remain shut."
I've got Skyrim for pc, but it might be that I never met Yngol's shade and maybe there was a Draugr Deathlord of something similar. So that's why I don't even remember to have looted it!