hi all
in my system around 73gb(pc-desktop)
i have,1 primary partition(windows)-25gb, 1-extended partition(remaining gb)
3 logical partitions were there in (under) extended partition
in one of the logical partition is d:drive.
in my hard disk d: drive is -/dev/sda5
----------------------------------------------------------

previosly i was fat -file system , (d:drive-/dev/sda5),
i remember i changed the d: drive(d:drive-/dev/sda5) file system to ext4file system ,with following command using terminal

--------'sudo mkfs -t ext4 /dev/sda5'----------
after doing(changing the file system)this one ,i couldnt see the d:drive data
---------------------------------------------------------------------
By doing that
1q) did i reformatted the partition? i think the new filesystem(ext4) has no knowledge of the data that was on it when it had a FAT filesystem.



2q)how to do undo operation,i tried to change the filesystem type to fat/ntfs in terminal using command --sudo mkfs -t FAT /dev/sda5.

result:its showing text message-'mkfs.FAT: No such file or directory'(not in single quote)

i had very imp data in d:drive
plz provide good solution for this

Yes, you reformatted - and certainly ext4 has no knowledge of the prior data.The only "good" solution is to restore from your most recent backup. Being "very imp data", you of course have a recent backup.
Other than that, search LQ for testdisk, photorec and forensic. This comes up all the time.
Recovery won't be simple, or probably complete.