Anyway, templates, languages, classes, you can always reverse what you done or restore the defaults either from the package or our demo.
Everyone should be very familars with very known safety rules which the first one is to backup your website before each change, a website working live like yours should also do fulll backup everyday. Cpanel has a good website backup button in it one click, Plesk has automated backup that can also be setup.
You mean Javescript error code, this means you messed up the JS code on page. Simply restore the default one, then redo your customization step by step without removing any JS code unless you know what you are doing.