I power up my mac air and no screen appears just makes the start up sound blank screen

Go step by step.

http://support.apple.com/kb/TS1365

Note: Steps 5 and 6

Step 5

Reset SMC. http://support.apple.com/kb/HT3964

Choose the method for:

"Resetting SMC on portables with a battery you should not remove on your own".

Choose the appropriate method.

Best.

Connect to an external monitor (you'll need an adaptor). If you see nothing your GPU/logic board is shot. If you see a normal display your onboard video chain has a problem (most likely a bad/shorted cable) but could be a bad backlight or backlight controler or the display itself.

AHT or an Apple Genius can help determine what is the issue.