(I have no idea what your technical expertise is like; please don't take offense to this.) Have you noticed whether the blinking cursor has changed from a vert

(I have no idea what your technical expertise is like; please don't take offense to this.) Have you noticed whether the blinking cursor has changed from a vertical line to a block? If it has (or, maybe, even if it hasn't, depending on OS + other factors), it's possible you've accidentally hit the "Insert" key on your keyboard, which toggles whether text is inserted or overwrites other text. It's an ancient feature that very few people consciously use any more and still results in the occasional call in to our shop from a bewildered client. Hitting the "Insert" key again should fix the problem next time.