I've read the following two solution:

Kernel Panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

kernel panic not syncing vfs unable to mount root fs on unknown block oo swapper not tainted

,but still got the same problem. Here's my full error code:

[1.902269] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[1.902299] Pid: 1, comm: swapper/0 Not tainted 3.2.0-29-generic #46-Ubuntu
[1.902325] Call Trace:
[1.902348]  [<ffffffff81640ec8>] panic+0x91/0x1a4
[1.902373]  [<ffffffff81cfc01e>] mount_block_root+0xdc/0x18e
[1.902400]  [<ffffffff81002930>] ? populate_rootfs_wait+0x300/0x9d0
[1.902426]  [<ffffffff81cfc257>] mount_root+0x54/0x59
[1.902450]  [<ffffffff81cfc3c9>] prepare_namespace+0x16d/0x1a6
[1.902476]  [<ffffffff81cfbd63>] kernel_init+0x153/0x158
[1.902501]  [<ffffffff81664034>] kernel_thread_helper+0x4/0x10
[1.902527]  [<ffffffff81cfbc10>] ? start_kernel+0x3bd/0x3bd
[1.902551]  [<ffffffff81664030>] ? gs_change+0x13/0x13

I don't know what these error messages mean at all, but I hope that these will help to solve this problem

I'm running Ubuntu 12.04.

Thanks for any advice.