How to recover lost/deleted files/data from USB Hard drive in ubuntu 10.10

foremost may help, your backups will definately restore them for you...

You could try "testdisk", but recovering data is very tedious.
I'm surprised that Andrew didn't tell you you must have made a backup...
If you need more help, first don't use the disk and explain what happened.

I do not know the exact cause for this. I have copied some data from my laptop to my portable hard drive 8-10 days back. After that , 2-3 times while the USB hard drive was connected to the laptop, it just froze(blank screen, mouse locked). nothing really worked out so i had to force shutdown it by using power button.
Then i rebooted the system and while doing this i did not remove the USB drive thinking that i might end up losing data if i do so. Each time i could see my data.

But now,a day or two before, when i tried to copy some data from my laptop to my hard drive it threw "error splicing file" and did not copy the file completely. After that no file was copied completely. and incomplete copied files are also not getting deleted. "error deleting file" is the message that i get.

whereas files which were there before this happened are fine. i can delete or read them without any issue. Also there was a folder on that drive which now i am not able to see. apart from that most of the data is fine. so i wanted to really check whether there was any folder which i ,now, believe that i have lost and if yes then i wanted to recover it as well. it's a 1TB hard disk and has more than 450GB free space yet unable to use the drive.

The file system is probably corrupted. And you could continue to loose files...
Boot on Ubuntu, plug disk, then System->Administration->Disk Utilities (or Gparted).
Unmount the disk and check its file system.
Then mount again the disk and open some folders on it.

If you really want to recover files, do any write on it and use testdisk. This software will try to recognize some files (firstly multimedia files), but you will not have file names, neither folder tree. So if you have many files, sorting them will be extremely tedious (I abandoned when I try it). And some files will not be properly recovered.

The community documentation is a highly important and very useful informational resource for data recovery in/with Ubuntu:
https://help.ubuntu.com/community/DataRecovery