Bad plugin or incompatible plugin. Always add one plugin at a time just in case it messes up your site. If you add a number at once, you won’t know which one is the culprit. Sometimes it can be that two plugins don’t work well together but are fine on their own.

Plugins that take up too many resources or too many plugins. When I was on a shared server with Hostgator, they shut down my site because I was using too many resources. They made me deactivate a number of plugins that they said were a drain on a site’s resources.

Web crawlers. I literally just went through this nightmare 2 days ago. A web crawler from 80legs.com (used by PR and Advertisers according to their site) sent over 300 bots to my site at one time! Even Google would only crawl a site like mine ONCE every 24 hours. Why they had 300 bots on my site I do not know. We told them to lay off and put me on the do not crawl list and my host blocked them.

Hotlinking. I dealt with this issue the day before the web crawler debacle! This is where someone takes an image off your site and puts it on their site without saving it to their own host. I had my grab buttons saved to my server (silly me) and a popular site grabbed my button still hosted on my server and it crashed my site.

Too much traffic. If you get a surge of traffic, it can bring your site down. For instance, one of your posts goes viral on Stumble Upon or you have an extremely popular giveaway that everyone is trying to enter at the same time.

Your hosting plan isn’t enough for your site’s resource usage. You get lots of traffic or use many plugins, you might find your site won’t function properly on a shared server. My site has a large amount of posts so it was too much for the shared server at my host to handle. That’s why I moved to VPS in March. Those unlimited hosting plans are never really unlimited. Read the fine print.

Your hosting company isn’t the greatest. I had a horrific experience with Fat Cow where my site was going down almost every day last year.

Shared server. If another site that you are sharing your server with is taking too many resources, it can cause your site to be slow or go down.

Hacking/malicious pinging. A friend of mine had her blog hacked by an adult website who was stealing her bandwith! It’s terrible that people do that to others.

Your domain expired. Some registrars don’t email you reminders so make sure you keep tabs on when your domain expires and renew! You don’t want to risk losing all your hard work if someone buys your domain.