I'm unable to start notebook in ubuntu 10.10 amd64

edit

I have just managed to install ubuntu and then sage a few days ago. Therefore I am an absolute beginner.

When I was using sage, due to external power issues, the computer suddenly switched off and after that, I am unable to open sage notebook. Its showing an error.

----------------------------------------------------------------------
| Sage Version 4.6.2, Release Date: 2011-02-25                       |
| Type notebook() for the GUI, and license() for information.        |
----------------------------------------------------------------------
sage: notebook()
The notebook files are stored in: sage_notebook.sagenb
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

/home/bhavanishankar/.sage/<ipython console> in <module>()

/home/bhavanishankar/Applications/sage-4.6.2-linux-64bit-ubuntu_10.04.1_lts-x86_64-Linux/devel/sagenb/sagenb/notebook/notebook_object.pyc in __call__(self, *args, **kwds)
    215     """
    216     def __call__(self, *args, **kwds):
--> 217         return self.notebook(*args, **kwds)
    218 
    219     notebook = run_notebook.notebook_twisted

/home/bhavanishankar/Applications/sage-4.6.2-linux-64bit-ubuntu_10.04.1_lts-x86_64-Linux/devel/sagenb/sagenb/notebook/run_notebook.pyc in notebook_twisted(self, directory, port, interface, address, port_tries, secure, reset, accounts, require_login, server_pool, ulimit, timeout, open_viewer, sagetex_path, start_path, fork, quiet, subnets)
    403     if open_viewer:
    404         "Open viewer automatically isn't fully implemented.  You have to manually open your web browser to the above URL."
--> 405     return run(port, subnets)
    406 
    407 

/home/bhavanishankar/Applications/sage-4.6.2-linux-64bit-ubuntu_10.04.1_lts-x86_64-Linux/devel/sagenb/sagenb/notebook/run_notebook.pyc in run(port, subnets)
    366                 checkPID(pidfile)
    367             except SystemExit as e:
--> 368                 pid = int(open(pidfile).read())
    369                 if str(e).startswith('Another twistd server is running,'):
    370                     sys.exit("""\

ValueError: invalid literal for int() with base 10: ''