how to find missing c drive

Solution
There should be more information there than just "your C: drive is missing"

I notice you didn't say the computer can't boot into windows. If you've booted into windows, Assuming your C: drive is your system drive (it most likely is), you may have been hijacked. Download malwarebytes on a separate machine, burn a bootable image to the cd (instructions are on malewarebytes site), boot your machie from that CD and run a scan on your system.
There should be more information there than just "your C: drive is missing"

I notice you didn't say the computer can't boot into windows. If you've booted into windows, Assuming your C: drive is your system drive (it most likely is), you may have been hijacked. Download malwarebytes on a separate machine, burn a bootable image to the cd (instructions are on malewarebytes site), boot your machie from that CD and run a scan on your system.
 
Solution