This could be any number of things and it’s hard to say without more details. How did you go about widgetizing areas of the site? What theme are you using? Have you don’t any custom css edits that may be causing the widget to be hidden? What plugins do you have installed and active on the site, they may be conflicting with the theme or the widget?
I hope this gives you some places to look that should help diagnose the problem. But without more info, there’s really no way to nail it down yet.