But without knowing what you've done or what the problem is exactly I'd only be wildly guessing...

What does your shop do now? Blank page? Report some sort of error? Not load at all?

Have you checked the logs (/var/log/apache2)? What do they say? Have you enabled debugging (I'm unfamiliar with Prestashop but most software allows you to set debugging mode which gives details of errors, although perhaps we're not to that stage yet, if your site isn't even loading...!?)

Bottom line though is that if it's a Prestashop specific issue then the Prestashop community is probably where you'll get your best answers...