You probably shouldn't have registered an LLC if you don't know what you're doing. It's more complicated than a sole proprietorship, and there are important filing requirements that must be handled. If you didn't renew it every year, it was probably admistratively dissolved by the state. There should be a state website where you can do a search on the public records to look up your LLC and check its current status.
I'm not sure how to restart an LLC, but you can simply contact the Secretary of State for your state to inquire about that. They'll be able to tell you.
Try a Google search on "[name of your state] secretary of state" to find the appropriate state website.