Someone has changed my default runlevel to 6 so whenever I start Ubuntu it reboots..
Can anyone help me by suggesting some solution how to change my default runlevel?
Ubuntu has recovery shell option, which is accessible through grub menu's Advanced Options for Ubuntu. Remember to remount system with
mount / -o rw,remount in order to allow changes to the system.
Alternatively, you can get to the grub menu, highlight
Ubuntu, press e to edit the entry. Add
init=/bin/bash, change default runlevel, reboot. Related link
There's also liveCD/ liveUSB recovery option.
Consult the ubuntu wiki for changing runlevels as well as booting to single user mode
Boot in single user mode (probably edit the boot command, adding a "1" at the end), and once the system is running, change the default runlevel back.