Is there any possible that the arduino board was destoryed so that I couldn't upload sketch? because I accidently switched the positive and negative power input of 7.4 v on the 5v pin and I heard something weird from the board and then I unpluged it immediately.
That may have wrecked the board.
Try this: connect Reset pin to GND, then the Loopback Test.
If that fails, the USB/Serial interface is dead, so you won't be able to upload.
If it passes, yet you cannot upload programs, the '328P chip is dead.
If either chip gets hot to the touch when the power is turned on, the chip is dead.
Since the board is all surface mount, you are pretty much out of luck if replacement is needed. Get an FTDI Basic and a Promini for your next round of testing until you more proficient making connections.