I am getting the black screen in startup

I am getting the black screen in startup

I am using Ubuntu 12.04 LTS. Actually i upgraded my system from 11.10 to
12.04 using terminal. after some days. I am getting the black screen while
starting the system. I am able to login after 4 to 5 times of rebooting
every day. some times it takes 10 times to login.
and i tried other answers in askubuntu.com it doesn't worked for me.
what am i supposed to do. Please do let me know