Hello how can i reboot my toshiba satellite c55..keep saying'reboot and select proper boot device'

Did you at any time change the boot order? Even if you didn't, I would go in there and make sure your hard drive is set as the first thing that boots.

If it is, and it still doesn't work, your drive may be going or gone.

If the device is under warranty still, then I would contact the manufacturer. If it isn't, then you can try and repair/replace it yourself or take it to a local tech.