Steps I followed to restore my system:-==>Since there was no command prompt available when the system started up without display I had to change "run level" of the system to get the command prompt . This is something like switching between different levels of operation in an operating system. You can use commands CTRL+ALT+F5 or CTRL+ALT+F3 . From what I have checked currently my display appears at CTRL+ALT+F7 in openSUSE 12.1 which is quite weird considering that I vaguely remember display appearing in CTRL+ALT+F6 or run level 6.
==>Then I had to login using my user name and password , typically the user name is same as that of your home folder and password is the one which you set when while installing the OS.
==>Then I checked the repos that were configured on my system using command "sudo zypper repos". I saw that all the repositories were enabled.
==>I had to refresh the repos after cleaning the meta data using "sudo zypper ref -f"
==>Finally i ran the distribution upgrade using "sudo zypper dup --download "in-advance"". Note that in-advance must be used with the quotes intact. Using the in-advance parameter all the required packages will be first downloaded before installation begins or zypper may choose to install packages after each package downloads.This process took about 1 hr.
==>The system was restarted using "sudo /sbin/shutdown -r now" after which the display started working again.
Note:-You can alternatively
==>Remove all the repos on the system using command like "sudo zypper rr 1 2 3 4 5 6 7 8 9" where 1,2,3... are the index numbers of the repos
==>Add the required official repos alone using the commands
sudo zypper ar -f -n non-oss http://download.opensuse.org/distribution/12.1/repo/non-oss/ non-oss
sudo zypper ar -f -n oss http://download.opensuse.org/distribution/12.1/repo/oss/ oss
sudo zypper ar -f -n update http://download.opensuse.org/update/12.1/ update
==>Finally do a distribution upgrade.