I had install Ubuntu 11.04 on two PC. On one everything work perfect, but another after some time turn off computer. I tried a lot of ideas(reinstall, openJDK 7(give just black screen), but all still as it was before. I hope on your help! Please! About PC(i don't remember all): AMD 5500; Nvidia (maybe) Zotac GT 450(512M) ram 2 GB And one more ask will instalation Sun JDK steal the trobel's?
Minecraft is not supported by openJDK. (I have learned this the hard way.) You need (well not need, but I haven't seen a running minecraft in openJDK yet) to install sun java, of which there are a myriad of ways (all of them buggy, none of them official) to install it correctly.
A fix for this issue might be an upgrade to 12.04 . It is an LTS, and is a lot less buggy then 11.04 (The first implementation of unity in ubuntu). A lot more stable, less buggy, etc. Other than that, there isn't a lot anyone can do to help.
See this website on a guide to installing sun java jdk 6-7 in ubuntu 12.04. I think either will work, and I also think that this should apply to 11.04. Alternatively, see this question for java jdk 6, this question for java jdk 7, and this quesiton for java jre 7. I would recommend a jdk of either version.