having ACER laptop with the following OS

Suse Linux Enterprise Desktop 10 – 2.6.16.54 – 0.2.8
Fail safe -- Suse Linux Enterprise Desktop 10 – 2.6.16.54 – 0.2.8
Ubuntu 8.04, Kernel 2.6.24 – 16 – Generic
Ubuntu 8.04, Kernel 2.6.24 – 16 – Generic (recovery mode)

Use ↓↑ to select which entry is highlighted
Press Enter to boot the selected OS, ‘e’ to edit the commands before booting, or ‘c’ for a command line.

SLED is working without hassle. curious to work with Ubuntu, selected Ubuntu and the system asked to upgrade to 10.04 and let it OK.
After finishing downloading, installing and cleaning and upgrading to 10.04, while restarting and selected ubuntu the following mess seen on the screen

Init : under head main process (2632) terminated with status 5
Libudev: udev_monitor_new_from_netlink : error getting socket : invalid arugment
Mountall: mountall.c:3204 : assertion failed in udev_monitor = udev_monitor_new_from_netlink (udev, “udev”)
Init: mountall main process (2635) killed by ABRT signal
General error mounting file systems
A maintenance shall will not be started
Control_D will terminate their shell and reboot the system
Give root password for maintenance
(or type ‘Control-D’ to continue)

Typed root password and the screen looks
root@frm-laptop:~#

Then selected ubuntu recovery mode, the screen shows

Begin: running /scripts/local-premount….
Kinit: name_to_dev_t (/dev/disk/by_uuid/71e12466-3ac4-4790-b93e-a37b35963cd3) = sda (8,5)
Kinit: trying to resume from /dev/disk/by_ uuid/71e12466-3ac4-4790-b93e-a37b35963cd3)
[81.665184] attempting manual resume
Kinit: no resume image, doing normal boot
Done
[181.697443] kjourneld starting. Commit interval 5 seconds
[81.697475] EXT3-fs: mounted file system with ordered data mode
Begin: running /scripts/local-bottom …
Done
Done
Begin: running/scripts/init-bottom
Done
Init : under head main process (2665) terminated with status 5
Libudev: udev_monitor_new_from_netlink : error getting socket : invalid arugment
Mountall: mountall.c:3204 : assertion failed in udev_monitor = udev_monitor_new_from_netlink (udev, “udev”)
Init: mountall main process (2668) killed by ABRT signal
General error mounting file systems
A maintenance shall will not be started
Control_D will terminate their shell and reboot the system
Give root password for maintenance
(or type ‘Control-D’ to continue)

Typed root password and the screen looks
root@frm-laptop:~#

what is the solution?

10.04 isn't supported and hasn't been for while while I don't think that's your problem but it seems sorta pointless to fight to upgrade to that.

how to revert to old version?

download an install currently supported ubuntu, at least 12.04, or better 14.04
ubuntu lts has 5 years support, so 8.04 ran out of support 2013, and 10.04 just recently

i have downloaded ubuntu 14.04. how to install

try the official documentation
http://www.ubuntu.com/download/deskt...ubuntu-desktop

with your know-how, reverting might prove impossible.

maybe re-install from here:
http://old-releases.ubuntu.com/releases/

...but, i would recommend installing a very lightweight, but current distro. maybe AntiX.

finally installed ubuntu 14.04 lts after formatting

How does it work?

Regards...