grub problem
 

one day suddenly I find that my grub file has changed a wee bit it also included the following lines (menu.lst)
title Ubuntu 9.04, kernel 2.6.28-13-generic
uuid a1e41098-9fb1-4e41-8ae4-fa8ef6c21acd
kernel /boot/vmlinuz-2.6.28-13-generic root=UUID=a1e41098-9fb1-4e41-8ae4-fa8ef6c21acd ro quiet splash
initrd /boot/initrd.img-2.6.28-13-generic
quiet

title Ubuntu 9.04, kernel 2.6.28-13-generic (recovery mode)
uuid a1e41098-9fb1-4e41-8ae4-fa8ef6c21acd
kernel /boot/vmlinuz-2.6.28-13-generic root=UUID=a1e41098-9fb1-4e41-8ae4-fa8ef6c21acd ro single
initrd /boot/initrd.img-2.6.28-13-generic

as compared to just
title Ubuntu 9.04, kernel 2.6.28-11-generic
uuid a1e41098-9fb1-4e41-8ae4-fa8ef6c21acd
kernel /boot/vmlinuz-2.6.28-11-generic root=UUID=a1e41098-9fb1-4e41-8ae4-fa8ef6c21acd ro quiet splash
initrd /boot/initrd.img-2.6.28-11-generic
quiet

title Ubuntu 9.04, kernel 2.6.28-11-generic (recovery mode)
uuid a1e41098-9fb1-4e41-8ae4-fa8ef6c21acd
kernel /boot/vmlinuz-2.6.28-11-generic root=UUID=a1e41098-9fb1-4e41-8ae4-fa8ef6c21acd ro single
initrd /boot/initrd.img-2.6.28-11-generic

I cannot understand why did this happen?
Plz help!

What is different than before? These four lines? Basically it looks like you had a kernel update. So now you have boot menu items for two different kernels, each one with a normal boot up and a recovery mode boot up.

That's exactly what happened. When you downloaded updates, it updated your kernel, and didn't get rid of the old one. You can remove the old one, if you want, by using:
It will clean up your boot menu, also.