Sometimes when I'm deep in thought, I'll have accidentally held down the "Shift" key for longer than 8 seconds and inadvertently turned on the dreaded "Filter Keys." Then I can't type numbers, or commas, or periods! It's like a horrible cap lock that only affects symbols and numbers.
I managed to find a helpful link to save the day! To turn filter keys off again, hold down BOTH shift keys at the same time. I tried it and it really works!a