invisible text

[chrissieben]

Help, suddenly all my spreadsheets have invisible text throughout. Format remains and data is all there but not visible or printable.
I've checked all the obvious (to me) things including -

that all text is black, all background is white.
that there is no conditional formatting in place.
that there is no protection on.

Any suggestions please, this has happened across all files so it's not a one-off mistake in one file.