300 GBs lost after WIN7 partition deleting

I have an iMac with 1000 GB of memory. I made a Windows 7 partition of 300 GB with Boot Camp, but, when i tried to delete it with Bootcamp, it gave me an error. Then, the Windows partition disappeared, but the computer continued to tell me that i have only 700 GBs on OS X Partition, so, i lost 300 GBs.

I tried to format my OS X partition, but it didn't work.

What can i do?

(P.S Sorry for my english, i'm italian)