Re: site not loading--also not loading admin.php page
The immediate cause of the problem is that your site was encoded using Zend Guard/Zend Optimizer, and that newer versions of the system (version 5.3) are not backwards-compatible with older versions (version 5.2 and earlier). Zend has stated publicly that they are aware of the problem, and have no plans to rectify the issue. In other words, they've thrown the existing customer base overboard.
That leaves you with a "what to do about it" problem. If the script you're using is currently maintained (and I don't have high enough system access to check myself), then you should be able to get either an unencoded version or a version encoded for Zend version 5.3. I would assume that most vendors using Zend Guard for IP protection are aware of the issue, and will give you a re-encoded version of their software at no additional charge. If the development is your own, you'll need to re-encode or use the raw source code.
On the other hand, if you were using the old sitebuilder (SiteReptile) here to build your site, you may be out of luck -- unless you download the site as it exists and set it up on a local box with a 5.2 install, which should let you grab the existing pages as HTML from the browser. Tedious, yes. And the encoding incompatibility issue is why x10Hosting has dropped SiteReptile as their sitebuilder software.
“Beware of bugs in the above code; I have only proved it correct, not tried it.” --Donald Knuth
"It was as if its architects were given a perfectly good hammer and gleefully replied, 'neat! With this hammer, we can build a tool that can pound in nails.'" -- Alex Papadimoulis (on TheDailyWTF.com)