VB 3.6.x MAJOR GLITCH - forum randomly stops working

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Before you guys bark me out and tell me to upgrade to vB 4.0 - I bought it during the pre-sale but am waiting for 4.1 and old mods I need converted over.

I have been running vBulletin 3.6.x for over 2 years on my main site. But over the past 2 weeks, vBulletin has randomly stopped working twice.



The first pic is the site normally, the second pic is what happens when it goes down. This is totally random - it was working fine when I went to bed last night and then today I find out it was down for the past 14 hours. No admin changes have been made for ages so that's not it.

The first time this happened, I was browsing my site (not logged in) and it was working one moment and then it went to this screen sitewide. Can someone please tell me what's causing this? This site is my job so I can't afford to have these random vB glitches happen. The downtime is really hurting my SEO big time.

The only way this is fixed is that I restore all my system files that are automatically backed up each night (only the files, I don't need to touch the database, which goes to show that the DB has nothing to do with it).

Please, someone help...