Hi there, I got a problem with my external hard disk.

I formatted it in an ubuntu os then suddenly windows 7 doesn't recognize it any more..

Is there any possible solution to my problem??

Ubuntu probably formatted it in an ext2 format (or ext3/ext4 which are based on ext2), which is not handled directly by Windows. There are free drivers for windows that will let you read, but not write, to an ext2-based file system.

FWIW, you CAN create NTFS or FAT file systems on discs with Linux, which ARE directly accessible by Windows, including for write access.

I have formatted it in NTFS format but still windows can't recognize it.

Does it appear in Disk Management?
Control Panel -> Administrative Tools -> Computer Management -> Click on Disk Management on the left hand side