ChromeOS running netbooks are gaining popularity, reaching in the hands of average users. Unlike Windows PCs ChromeBooks offer extreme security and ease of use. There is no way your grandma can mess up with the PC by mistakenly deleting system files or doing something crazy. However, things may go wrong sometimes. As it happened to me -- when I got back from my US trip and tried to turn on my cr48 I was greeted by this message: "ChromeOS is missing or damaged".

Google has done an excellent job at helping out users to recover the OS with greater ease. I am reproducing some steps that I took to recover the ChromeOS. I use Linux so the instructions are for Linux users only.  
 
Requirements:
1. A USB Flash drive (4GB minimum)
2. A PC will Ubuntu/Linux installed
3. Internet
4. Confidence in working with command line
 
To recover the ChromeOS you will have to work in the terminal for a while, don't get scared it's extremely easy. It's as easy as writing an email.
 
1. Insert your Flash Drive to your PC and make sure to remove all other USB connected storage devices so that you do not mistakenly erase those.
2. Download the .sh file
3. Open the terminal and change the permissions of the file. Run this command

sudo chmod 755 /PATH_TO_THE/linux_recovery.sh  
 
Now run this command to download the recovery files

sudo bash /h/PATH_TO_THE/linux_recovery.sh  
 
This command willl prompt to choose the appropriate image. There are three types of Chromebooks in the market --  
1 - Samsung Chromebook Series 5 US-Wifi
2 - Samsung Chromebook Series 5 Non-US-Wifi
3 - Samsung Chromebook Series 5 Non-US 3G
4 - Samsung Chromebook Series 5 US-3G
5 - Cr-48
6 - Acer AC700-1099 Wifi
7 - Acer AC700-1529 3G
 
At the bottom of the options you will see this: Please select a recovery image to download:

You can enter the appropriate number there in my case it was cr48 so I entereted

Please select a recovery image to download: 5

This will start downloading the image.

Once the image is downloaded it will verify if you have the correct USB drive connected to the PC. There will be two options 0 for quitting and 1, 2, 3 for connected USB drive. As I suggested please removed all other USB drives to avoid any issues. If you have only on USB drive connected, the one you want to use for recovery, you will only see option 0 and option 1 with the name and size of the drive. Select 1 to choose the USB drive. In case you do have other drives connected carefully choose the number.
 
I found 1 USB drive.  We need one with at least 927MB capacity.
0 - <quit>
1 - Use /dev/sdd 8019MB Lexar    USB Flash Drive
Tell me what to do (or just press Enter to scan again): 1

It will once again confirm that this is the drive you want to format:

Is this the device you want to put the recovery image on?
  /dev/sdd 8019MB Lexar    USB Flash Drive  
You must enter 'YES' (all uppercase) to continue: YES
 
Well, unlike Windows where users keep messing up, Chrome once again asks to confirm:

I'm really going to erase this device. This will permanently ERASE
whatever you may have on that drive. You won't be able to undo it.

/dev/sdd 8019MB Lexar    USB Flash Drive 

If you're sure that's correct, enter 'DoIt' now (case is important): DoIt
 
Once you enter DoIt, it will erase the drive and install the recovery image. You see the process is so simple. Now, unmount/eject the drive and insert it in your ChromeBook. The chromebook will detect the recovery image and automatically start the process.

Once the recovery is done wait for it to restart the Chromebook, leave the USB Plugged. If everything goes well, you will be greeted by this window. Just follow the instructions and you will have your ChromeBook running.