I am using Ubuntu 11.04 with Windows 7. Ubuntu was working nice but now when I choose Ubuntu 11.04 from boot loader, it is not opening. Just Purple screen is coming but then nothing change. I can't see login menu, it doesn't pass to the login menu. What should I do? Has anyone this problem?

Thanks for your answer. I did the steps you wrote Frantique. But now I have a black screen and then nothing. When I choosed ubuntu 11.04 from bootloader, first purple screen is opening then after 1 second it is going to a black screen. And I have nothing again. I can't write anything. Just I have a black screen. It is not openining.

Before those problems they work together nice. There aren't any errors. Lets I tell you what did I change: I installed docky, and theme of MACOSX. They were working nicely. And I tried to install a nice calendar. To do this installation I changed a litle bit the file sources.list. That calendar program was working succcesfully too, after installation. The theme MACOSX was working succesfully there weren't any problem. After some days maybe meeks I had that problem which I asked in the above. Now when I tried to update I have a lots of errors. Like: Failed to urls... can't access archive something... .... Failed to urls... can't access archive something... .... ..... Failed to urls... can't access archive something... I hope you will solve the problem. I want my MacOSXUbuntu back :)