Perhaps overtype mode has been turned on accidentally.
If you're using Word 2003 or earlier: look for OVR in the status bar at the bottom of the Word window; if you see it, double-click it to turn off overtype mode. Or press the Insert key.
If you're using Word 2007: click the Office button, then click Word Options.
Click Advanced.
Clear the check box "Use overtype mode" in the Editing options section.
Click OK.
If you're using Word 2010 or 2013: select File > Options.
Click Advanced.
Clear the check box "Use overtype mode" in the Editing options section.
Click OK.
