You may have inadvertently clicked on the HTML button (source code) at the top of the editor window, which looks like this: < >

All this does is reveal the code behind the text. It turns the background of the editor black, rendering the letters white.

If the above isn't true in your case, you probably pasted in some text from another source. We do not recommend doing this because, oftentimes, pasted-in signatures link back to code that isn't public (it's usually from inside a larger company's network). When that happens, the links break, and your signature doesn't render.