Problem with my bicycle that I NEED to fix!!!!?

hey, i need help with my bike. i'm a first time bike user, and i really don't know the names of ANYTHING. i was biking one day and my hand accidentally hit the gear on the left handle which moved the number up to 2. i know it's not the speed because that's the gear on the right handle, it has 7 speeds. i moved it one more up on purposed and found out it went up to 3. when i clicked it back to 1, my bike stopped and made some weird clicking sound, and now my pedals spin oddly, and they don't make a clicking sound. they just spin around and around, and don't even propel the bike forward. also, if you spin the pedals around about 3 times, it stops and doesn't move anymore. i've tried clicking the gear on the left handle again, but nothing seems to work. I need my bike to take me to school, friends houses, etc. I got the bike at Target for $200, and it's been working really well for about 2 weeks, no problems at all. HELP...PLEASE!!!