I have had it happen to me 4 times this year on my 2lt-E's alt

.... i now keep a spare working alternator at home because of it. picked it up cheap off trademe!
as others have said its normally just shit getting in between your brushes so they do not contact properly, if you been down to the waimak for a play and been in mud around alternator level that silty mud normally gets stuck in there, sets like concrete

if you are capable of doing it yourself i would take it apart and give it a clean
brushes are really cheap too do if you do them yourself, or i paid an auto electrician to replace my brushes on the alternator drive in drive out for $80 odd dollars