Macintosh HD : password is not working

Hello !

I have an MacBook Air 11-inch. When I'm on the application "Disk Utility" it is stated that the Macintosh HD disk capacity is 120.12 GB and I only have 5.92 GB left. This leaves me very little space on my mac, yet I have very few files (I almost put them all on an external hard drive: Seagate). So I decided to check the condition of the disk thinking maybe that there is a mistake (still in the "Disk Utility" application) and it appears to me that the: Disk Utility has stopped to check "Macintosh HD"; This disk needs to be repaired using the hard drive recovery. Restart your computer, hold down the Command and R keys down ... When the window Utilities OS X appears, choose Disk Utility.

I made the steps mentioned (Command + R keys and Disk Utility). Except that once arrived, Macintosh HD is locked and asks for a password to be unlock and therefore perform the disk check. I insert my password several times without success. With the clue, I know that the password that I entered is correct, plus it is the same password used to open the session at startup (with the same clue). So I change my password and the clue. Result: To go into my session: the clue has been changed, the password is correct everything is good. Once arrived to unlock the drive: the clue has been changed but the password is still wrong!

Your help would be very valuable to me! I need to quickly make space for all my study documents!

P.s : Sorry my english is not perfect (I'm used to speak/write in french)

Thank you very much !