[Theme: Optimizer] Uploads but no theme shows

Hi, I’m using Responsive theme at this moment, since 2012, but for some reason a couple of months ago my site is very slow, and some features (Plugins) I need to use on my site, now aren’t working properly and others nor working. I then created a sub-domain using a theme called Optimizer, which is working fine with all the plugins I’m using on my main site, but which are not working on my main site. I tried to replace my Responsive theme with Optimizer theme, but the theme is a total mess… there’s no theme on it and seems whatever I do, nothing works.

How can I get my WordPress site back to the default site, so that I can change it to Optimizer theme?

Thanks in advance

Deon