I downloaded a texture pack and instead of it adding on to the textures i could use, it replaced the default texture on my minecraft.  I can no longer play on a browser, it just has a frozen screen with a partially loaded bar.  If i download another texture pack, i can't play on either the browser or downloadable version.  So now I am stuck with this texture when i would rather play on the real default.  And my game has been laggy (too laggy to even open a door) lately, but i guress that's another story.

Please help, and explain in great detail, as i am slightly computer-retarded.