It's difficult to say without looking at your project!
It could be that you've accidentally placed the sprite on a layer which is below another layer containing a large image (note: higher layers override lower ones). It could be that somewhere in your event sheet you've accidentally added code to make your sprite invisible. And it could be a whole bunch of other stuff!
Perhaps, if it's none of the above, you could post a capx, so that others can check it, and help you to solve the problem.