Mac Shut Down In Middle of Mountain Lion Upgrade, Crashes on Startup Now!

[itsthemacaddict]

I was installing Mac OS X Mountain Lion and accidentally shut down the computer while it was updating (MacBook Air). Now, when I turn it on it crashes with the 'You need to restart your computer..." message and at the top says "Unable to find driver for this platform" and Mac OS Version: Not yet set.

I tried booting from the Recovery HD, but then it wouldn't let me re-install Lion because it thinks Mountain Lion is installed and it can't downgrade. I went into Disk Utility and backed up the Macintosh HD to an external hard drive, hoping that I wouldn't loose any data.

How do I re-install Lion/make my Mac work normally again?? Thanks!

 

[newfoundglory]

1. turn on mac
2. hold down opt/alt to select a boot device (you will need a [Mac] OS X installer on an external drive or DVD)
3. format using disk utility, and re-install

 

[monkeybagel]

If it is a 2011 Air do Internet Recovery.

Hold Cmd+Option+r during boot and follow instructions.