Why did Unity stop working after I upgraded to the 3.4.3 mainline kernel?

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http://askubuntu.com – I installed the latest Linux kernel (3.4.3). After rebooting, it says I am in Ubuntu 2D. How do I either go back to my previous kernel, or fix this problem? Thanks. $ uname -r 3.4.3-030403-generic (HowTos)