Hello. My name is***** can help you with your question.
Before I say anything I will say that you could send this out to a data recovery service which, although expensive, has a very good chance of getting your data back. There is some chance that you can recovery yourself, but following any instructions I give you could actually make the situation worse. So with that caution, the first thing I would do is to boot up your computer and then go to a command line and for the drive with the problem type the following
chkds d: /f
and in this command replaced the with whatever drive letter this drive is being seen as. If that doesn't work and the drive is not being seen by the computer at all, that is a more serious situation that will probably need the involvement of a recovery service that can repair the hardware parts of the drive.
After the command I gave you, it may take several hours to run if it does find errors but it should fix them. Do not stop it once it has started so you may want start check at night and just check on in the morning.