[Mothy]

Common problem reported to help desks - and one I have helped my father-in-law with countless times.

You have typed CTRL-a instead of Shift-a for a capital A at the start of a new sentence.

The CTRL-a selects all the text in the document which is the replaced by the next letter you type. The single letter you have on the screen is the one following the A which replaced all the highlighted text.

The simple fix a most people have already suggested is to 'Undo' - use CTRL-z or just the undo button - but you must do it straight away - DON'T save first.

Tim