Troubleshoot Overtype
Why is the text disappearing as I type?
By default, any new text you enter in the middle of a line is inserted between existing text. If existing text disappears as you type, Overtype mode may be turned on. On the Tools menu, click Options, click the Edit tab, and then clear the Overtype mode check box.
You can also double-click OVR on the status bar to turn overtype on or off.