Why is my computer changing letters as I type them?

I have run into something similar when using a lap top. The keyboard had a function key. I believe it was labeled FN and it was blue at the bottom. What had happened is that I accidentally hit a combination of the function key and one of the F keys at the top. (F1-F12) That put the keyboard into a different mode. I'm sorry I'm not sure of the fix as I now have a desktop and I forgot exactly what keys where pushed. I hope this helps.