The disk you inserted was not readable by this computer

Help! I've been using my iStorage Diskashur for the past month with no problems at all. Since using it I formatted it for Apple use and have all my files on there. Since yesterday however when connecting it to my machine it gives the error message 'The disk you inserted was not readable by this computer' with options to 'initialize', 'ignore' and 'eject'.

I'm panicking that all my files are lost and I can't think why its happened. I've not changed anything!

Would really appreciate some help so I can recover my files