Check the document resolution. If it's set to something extremenly high or low, your type could be hard to see.
Then open the character and paragraph panels and check the values in them (or just reset both of them).
The Type tool makes the size of the type you selected in points the proper size on the document. For example, 72 point type is supposed to be about 1 inch high. How many pixels/inch is your document set to? What point size type are you specifying? What is your zoom level? What I'm getting at here is that you may have set up a condition where the type is simply too small to see on your screen.
Also, what color is the Type (e.g., is your foreground color white and you're trying to type on a white background)?
Try returning the Type tool options to default by making the Character panel visible, clicking the little icon at the upper-right, and choosing Reset Character.
Finally, make sure your video drivers are up to date from the web site of the manufacturer of your video card.
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