Entire Site Down with this error:- Parse error: syntax error, unex..

ok what is this please !!! its just happened for no reason at all and taken entire site down

Parse error: syntax error, unexpected T_CONSTANT_ENCAPSED_STRING, expecting ')' in /home/socialmu/public_html/cache/sys_options_acd5c74b4b739d910d9de3585324166e.php on line 680

regards

Clear your /cache (except .htaccess) and /cache_public directories and see if that resolves it.

 what would cause this please and, or how do I prevent it from happening again ??

That should have caused because of some error in the cache creation. If your problem comes again, you need to look the files for any change from the original build package.

 its relatively evident what caused it in regards to the site going down. what the inquiry was, is what caused this, and how to prevent it from happening in the future?

 the answer to that lies in the deleted file which was a cached instance of something in the sys_options table that was not liked well by the server. what that means, is that something was modified on the site, which instantiated a query on the database. based on the query on the database from the modification, your cache file was deemed inadequate. 

most probably is that the file had some issues, so in the event you reversed your changes on the database, the file was not writable, in the cache directory. 

you can disable database cache and file block cache from your admin panel and this may prevent situations such as this. 

Disabling database cache is not a better solution as this will slows the entire thing. So, he should check the file that generates the database cache for any issues. As the issue is not anymore after deleting the cache, it was not because of the cache generating file. The problem is he should have tried to edit the cache file manually.