Hi there Fabricio_1,
Do you have two separate hard drives, or you have two partitions on the same hard drive?
Anyway, I would suggest you to back up the data stored on your accessible partition/drive. Once you do that, you can go to Disk Management and see if the drive appears as unallocated. In case it does, then you may need to use some partition recovery tool.
If you manage to retrieve your data, then you can check the drive's health status out, as it could be failing: http://www.tomshardware.co.uk/forum/282651-32-best-diagnostic-testing-utility