try do this and see if it can mount

root@ubuntu:~# fdisk -l
root@ubuntu:~# mount /dev/mmcblk0p1 /mnt/
root@ubuntu:/# touch /mnt/test1
root@ubuntu:/# cd /mnt/
root@ubuntu:/mnt# ls

test1 file should be created.

The quick to restore everything back to normal is to use USB Recovery tool.  You can find it here.

http://www.marvell.com/products/embedded_processors/developer/kirkwood/sheevaplug.jsp

Try these commands at the uBoot prompt; it skips bad blocks if encounters any.

>>setenv bootargs 'console=ttyS0,115200 mtdparts=nand_flash:0x400000@0x100000(uImage)ro,0x1fC00000@0x500000(rootfs) root=/dev/mtdblock2'
>>setenv bootcmd 'nand read.e 0x800000 0x100000 0x400000; bootm 0x800000'
>> saveenv
>> reset

Have you tried to use the driver from the CD?  You can also fetch it under resource --> download from openplug.org site