Magento website suddenly has 'No data received' Error code: ERR_EMPTY_RESPONSE

Question

We have Magento running on an eCommerce website an the last few days it seems that some really important pages are simply not loading!

Majority of the pageson the site load fine however if a user is "logged in" and they try to access the cart checkout page, by far the most important page, they receive this very nasty message shown below in the image...

As far as I know, no changes to code have been made the last few days so I do not even know where to look at this point for a solution.

A Google search shows that many people had had this problem before, mostly on WOrdPress sites but there never is a solution posted!

I am hoping someone can point us in the right direction as this is a major problem on this site right now if a user cannot checkout.

The other page that i noticed it is happening on is the page to create a new user account, once you fill the form out and hit submit, it shows the page shown in image as well.

We are running Apache on Ubuntu 12 believe. We also have Varnish Cache if that means anything.

Please help!

Also here is what I get for the same page in FireFox...

Answer 1

As mentioned by others, try flushing the APC Opcode cache.

You can do that by adding the following lines to your magento/index.php right at the top of the file to clear the cache

apc_clear_cache();
apc_clear_cache('user');

WARNING

As one cause of the problem can be a corrupted Opcode cache you should only use this once (add the code, then call the page) and comment it out afterwards, otherwise you won't have any performance improvements by the APC, maybe even a degrading performance as the cache will be filled with every page call.

So, add the code, call the page, then comment the code out. (if it didn't work, comment it out and search for another solution, DON'T leave the code in the index.php)