Hi,\r\ni have a problem with my macbook air when

Try restarting your computer while holding down the X key. This may force the computer to start up using your Mac OS X system. If it does, release the X key, then open System Preferences and click Startup Disk. Make sure your Mac OS X system is selected. If you are unable to start up using the X key, restart your computer and immediately press and hold the Option key. Release the Option key when icons start to appear in the center of the screen. Select the Mac OS X system disk, then click the forward arrow to continue starting up.
If the above attempts to reselect your Mac OS X startup volume are unsuccessful, try resetting PRAM. Shut down your computer, then turn on the computer and immediately press and hold Command-Option-P-R until you hear the startup sound for the second time. Open System Preferences, click Startup Disk, and select your Mac OS X volume.
Hope this helps. If none of these options fix your problem, bring your computer to an Apple store, they are always very helpful and almost never fail to fix the problem!