Generally speaking, when things don't show up in the main preview area, it's because of display driver problems, though in the case of font problems there is also the possibility of fonts just being badly crafted.
The first and best advice, assuming you're on a PC, is to go to the web site of the maker of your video card and download and install the latest display driver release for your hardware and OS.
Also, there are a number of display options you can access through the Preferences - Performance settings. Specifically, try changing the Drawing Mode and options in the [Advanced Settings...] section. Make SURE to quit Photoshop and restart before testing changes there, or you won't be testing what you think you're testing.
Finally, I'd suggest going through the font troubleshooting steps in this page by Adobe:
If Noel's suggestions do not fix things for you, then I would first look at the Character Palette, and determine what font, what font size and what font color is set there. If you do not see an issue, say a white font on a white background, or a tiny font, then I would reset the Type Tool.