first thing, si abcklup the site, even though its broken, I recommend joomlapack
then
Did you use quickstart or did you just install the template
are you using joomla 1.5.x or 1.0.15
If you used just the template and not the quickstart
go to your template manager and assign another template as the default,.
the delete the mesolite template.
then reinstall the template
that should give you fresh files again
go to template manager and make mesolite the default,
it should be back to the original
and now you should be able to mess it all up again 🙂
thats how we all learned.
now, if that did not work, you can delete the whole site and install the quickstart package and your can astart all over again.
or if you want to keep your content items, you can install the quickstart and use you exiting database and NOT INSTALL sample data. This way it will be a clean install and your content will be there, you will have to reinstall any components, modules and plugins, but that will only take a few minutes
but, its always best to start fresh and in an environment where you can start over from scratch until you get the hang of joomla.