What is wrong with my bike?

[Pulverizer Aron]

I recently bought a diamond back moutain bike and ive had it for about a week.

Yesterday I was riding it and something happened... The bike wouldn't go when I was pedaling...

The chain was still on and I was even able to switch gears, but the back tire doesn't move when I pedal.

 

[JO-CO]

Sounds as if your cassette hub is stripped. You should take it back to where you bought it for service.