Not really sure how to solve. Here are some ideas.
The left hand side bar would be handled by the css and php in the style sheet and templates under appearance - editor in the dashboard. If you know html or css coding and tried to put some directly on your page, check the coding. An extra <div> can mess up your layout. Just because you are new to WP, doesn't mean you are new to coding. That is why I mentioned it. I'm going to assume you didn't mess with any of that.
The problem should lie with a plug in or script you entered.
I believe you are using the default theme twenty eleven. I know not all plug ins will work with all themes. I would think most if not all plug ins would work with the default theme, but maybe not.
Check the pagination plug in first. I haven't used a pagination plug in, make sure you entered the information in correctly. It could be as simple as that. It could have had a bad install, or just not working with the theme or other script on the site. You might have to use a different plug in that basically does the same thing. That will hopefully solve the problem of not being able to go to the next page.
Who knows it could be interfering somehow with the left side bar also. If the pagination didn't interfere with the sidebar, check the rest of your plug-ins also. At minimum deactivate them, and see if there is a difference. At maximum uninstall, check and then reinstall. I don't think you would have to go that far.
The other thing to check is any scripts and java scripts you put on your products page. You could try taking a product page, removing the script, and seeing if it solves the problem. If not, you can replace the script, and check the next one.
Those are the simple things you can do. Other than that, it gets into coding.
Maybe someone else has some ideas.