If the document is still open, press Ctrl+Z (Undo). Most likely you pressed
Ctrl+A (Select All), and the next character you typed overwrote all the
content. If you have closed the document without saving, then just reopen
it. If you saved and closed, then you're out of luck unless you have Word
set to save a backup copy, in which case the previous version will be
available as Backup of <filename>.wbk.