Ubuntu 10.04 does not boot after encryption

I own a desktop computer system and use UBUNTU 10.04 O/s. Prior to this I had Windows 98 but I then replaced it with the UBUNTU O/S. I share a room with my colleague but I do not trust him much . So during O/s installation I encrypted it to protect it from unauthorized access. But during that process something went wrong in the root folder, now the O/s is not booting up.I still have the encryption key with me. Is there any way to recover my documents from the drive? Can anyone assist me to get back those documents ?

As you said you were protecting it from your colleague so that he could not get his hands on your system and the documents stored in it and decided to encrypt the O/s. But I think in that process you accidentally deleted some of the important and necessary files from the root directory of the O/s that are most required to Boot an O/s. So even if you have encryption key with you , I do not think it will help you to fetch and retrieve your documents back to it's previous state. So I would recommend you to reinstall the operating system again.

If you created encrypted volumes during the installation and assigned them mount points, you will be asked to enter the passphrase for each of these volumes during the boot. The actual procedure differs slightly between dm-crypt and loop-AES.
dm-crypt

For partitions encrypted using dm-crypt you will be shown the following prompt during the boot:
Starting early crypto disks... part_crypt(starting)
Enter LUKS passphrase:
In the first line of the prompt, part is the name of the underlying partition, e.g. sda2 or md0. You are now probably wondering for which volume you are actually entering the passphrase. Does it relate to your /home? Or to /var? Of course, if you have just one encrypted volume, this is easy and you can just enter the passphrase you used when setting up this volume. If you set up more than one encrypted volume during the installation, the notes you wrote down as the last step in the section called “Configuring Encrypted Volumes” come in handy. If you did not make a note of the mapping between part_crypt and the mount points before, you can still find it in /etc/crypttab and /etc/fstab of your new system.
The prompt may look somewhat different when an encrypted root file system is mounted. This depends on which initramfs generator was used to generate the initrd used to boot the system. The example below is for an initrd generated using initramfs-tools:
Begin: Mounting root file system... ...
Begin: Running /scripts/local-top ...
Enter LUKS passphrase:
No characters (even asterisks) will be shown while entering the passphrase. If you enter the wrong passphrase, you have two more tries to correct it. After the third try the boot process will skip this volume and continue to mount the next filesystem. Please see the section called “Troubleshooting” for further information.
After entering all passphrases the boot should continue as usual.

There are some scenarios mostly in the Linux and Ubuntu operating systems where the encrypted volumes do not mount because of the wrong passphrase:

• The first case is when the root partition is not mounted properly, in such cases the boot process halts in between and the only way is to restart the system and try again.
• You can mount the encrypted volumes manually using the command # mount mount_point and you will have to enter the passphrase for the encrypted volume.
• If your system has any files which is not much critical then the system should boot and should be able to mount the volume manually just like the previous scenario. you will have to type #init at the shell prompt and press ctrl+D if asked for the root password.

Try to Boot the O/s as a Live User, once you do that you will be able to manually mount the encrypted partitions. Then you will be able to load the operating system module manually and you can recover whatever files and documents from there you thought you had lost. I know this is a bit tedious, you can also do it in the troubleshooting section once you open the terminal, you can type fdisk-1. Once the live-cd is booted you can mount the encrypted section and type correct password using the command "mount".