Your website could be down for any number of reasons, so there is no way to definitively answer this question. However, here are a few pointers:

Your Website Does Not Show Up At All

If you try to go to to your website and it does not pull up at all, here are some possible things to check or look into:

If you can’t pull up your website at all and your domain registration and hosting have not expired, contact your hosting company for help.

Your Website Shows Up, But It’s All Kinds of Screwed Up

This most likely means something is wrong with your website itself and NOT your domain registration or hosting. Unfortunately, there could be any number of things going wrong or broken. You will most likely need to contact a web developer to help you.

My Website is Hella Slow

If your website seems to load correctly but it’s just insanely slow, most likely you need a better host and/or optimization to your website to help it load faster. In either case, you most likely need to contact a web developer for help.