Yes, that’s a rather vague, all encompassing title. Trust me it fits. Sometimes your site all of sudden for no *apparent* reason. Stops working. Pages load slow, pages don’t load, you get spinning wait cursors forever. Something obviously is not right but all the normal debug and conflict testing doesn’t make any difference. Trust me there is a reason when things go wrong. Usually it’s due to upgrades or conflicts. Here are some things to try when something is wrong and there doesn’t seem to be any obvious reason.
First, make sure WordPress and all your plugins are up to date. Especially if you are using Gold Cart. WPeC and Gold Cart need to be in sync with compatible versions. The best practice there is to make sure they are all current.
If everything is up to date something more ominous may be happening. It’s time to scan your site for hacks.
Sucuri provides a web based test and will also list places you may be blacklisted http://sitecheck.sucuri.net
Additionally here is a page of malware scanning plugins https://wordpress.org/plugins/search.php?q=malware+scanner
If your malware scan does show your site has been infected it is strongly advisable to hire and expert to clean it up. A hacked site can be difficult to clean. We won’t begin to attempt to provide the steps needed to correct a hacked site.
WordPress has a great resource if you are going to attempt to do the cleanup yourself. http://codex.wordpress.org/FAQ_My_site_was_hacked
Securing your site and having a strong backup plan ahead of time will often save you from this headache.