website may have been hacked

I was told by a client a few weeks ago that it appeared my website has been hacked. As a novice with websites and development, I can't figure out what happened. I tried searching here for solutions and throughout the web - no luck. If anyone has any ideas / suggestions, I'd appreciate any help. Can it be fixed or does it need to be destroyed and start over?
This is throughout the site: "Strict Standards: Non-static method PermissionsCache::exists() should not be called statically in"

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PineCreativeLabs replied on at Permalink Reply
It appears you're using an old outdated version of Concrete (the theme you were using "Vanilla Yogurt" is the default for legacy Concrete). The old legacy versions aren't well supported anymore by many web hosts, as it requires an older version of PHP. So, it's very possible that your web host has upgraded their PHP version, which would break your site. In this case, you would need to completely rebuild with an updated version of Concrete.

A possible "patch" fix for this would be to edit the .htaccess file, and force the PHP version to use an older version. However, this isn't really recommended, as older PHP versions are now deemed as "deprecated", which legacy versions of Concrete are now considered fully unsupported and "end of life".
ConcreteOwl replied on at Permalink Reply
This can be fixed, I have sent you a PM