I need to restore uxtheme.dll onto my computer, but its not in my recyle bin. !! URGENT!!?

Long story short, I was trying to install a third party theme and followed a guide for such exactly (i can't really bother to find the url for it now) but it told me to rename 3 system files in the system32 folder, once of which being uxtheme.dll to uxtheme.old. I did so, restarted, and now nothing is showing up - no start bar, no desktop, not a thing. I can't even view it in Safe Mode, like one guide instructed me to do when I tried to fix the error. The only reason I'm actually on the internet is because I'm using my brother's computer. I cannot figure out what to do, I'm on the brink of tears. I cannot lose all the files on this computer. PLEASE help me ASAP :(