That depends very much on what happened to it, the file system it was using, and the platform you are using. As such, there is no single or comprehensive set of instructions that can be applied to such a broad question.
You can't recover passwords in Linux; you can only reset the passwords on various accounts.
There are many cheap ways to get Linux recovery help. One looking to find Linux recovery help should visit on the web sources such as Linux Certified, Relax and Recover, and CNet.
you can partition the drive.. to use both OS
I don't believe you can recover a file from a USB drive if you don't have the USB drive. I keep backups of the files.
By formatting the hard drive.
To format a partition that is not in use get a program called Gparted. If you need to format the whole drive you need to boot from a Linux CD.
you can find all the usb attached to linux by lsusbcommand in terminal !
It is not hard to recover your hard drive if you have a back up to your files. If you are willing to spend money you can fix your hard drive by visiting Geek Squad.
If you have a USB drive and your BIOS supports booting from a USB device then try that.
The most likely reason is that the drive was formatted with a Linux file system. Reformatting the drive to NTFS or FAT32 will make the drive usable in Windows. You could also install an ext4 driver in Windows to access the drive without reformatting it.
Please see the related link below for an example of running ubuntu Linux from a usb flash drive. Linux is installed on the usb drive allowing you to boot directly from it.
The typical way to recover lost data is to purchase a new hard drive and install it as a "master" hard drive with the old hard drive as the "slave". You can then use recovery software to try and recover data from the old drive. This process is best done with professional help.