It sounds like your "a" key has been remapped to display the symbol it does instead of the "a" you want somehow (via prank, glitch, cosmic event, flying monkeys....).
What is the delete key doing? Maybe it's related, or can be similarly fixed.
I found a Mac key mapper here: http://www.macupdate.com/app/mac/12004/doublecommand but since I do not have a Mac, I cannot test it to see if it'll let you remap your "a" key. Reviews are mixed, proceed at your own risk. Maybe try and find an alternative program if you wish.