The Insert (Ins) key is a key commonly found on computer keyboards.
It is primarily used to toggle (switch) between the two text-entering modes: insert and overwrite. In insert mode (most common), from the cursor’s current position characters are inserted as you type, pushing all future characters back. In overwrite mode, from the cursor’s current position instead of pushing the text, it simply overwrites the exiting text.
So, the fix is simple…just hit the Insert key once and it toggles back to insert mode.
Adapted from http://en.wikipedia.org/wiki/Insert_key