If you use WordPress long enough, it is inevitable that you will run into some sort of problem along the way.
The problem is, not all plugins and themes are going to work together, so it is usually some conflict. You normally resolve these issues by going through the process of elimination.
First, you try a different theme and if that doesn’t resolve the issue, you have to deactivate all of your plugins, then activate them one by one until the problem comes back.
This process can be a major pain if you have a lot of plugins installed. Thank goodness there is a new way, a better way, a more automated way to go through this process.