Switching between themes is quite common for WordPress users. Whenever you change any theme, not all the features of the older theme can fit in the new theme. Inactive Sidebar is the result of changing a theme. The sidebar that was available earlier in the previous theme will stay as an inactive sidebar in the new theme. Especially, those custom sidebar which is added by users and not by default can go inactive. This happens because the old sidebar is not registered for the new theme. Here are the solutions for the problem of how to activate an inactive sidebar in WordPress.