System not booting?

[OGFatal]

I upgraded my mobo cpu today and everything thing was working fine. I shut it down about 6 hours ago when I was out for dinner but now when I boot my keyboard and mouse don't work and I get a screen that says ' Reboot and select proper boot device. Or insert boot media in selected boot device and press any key.' I'm stuck for what it could be, any help is appreciated.

[player9050]

make sure your SATA is connected fine, maybe when you were changing cpu, you accidentally disconnected cable from HDD/SSD or mobo. Just make sure

[XxSwankeyxX]

Reset CMOS and unplug then replug bios battery.

[Andrei777]

Check connections to your hard drives/ssd, both power and data cables. If that didn't work, go into the BIOS and see if your main OS drive appears, if it does, select it as the #1 boot priority. Last thing I would try is a BIOS reset like XxSwankeyxX just said. Just wait like 10 - 15 mins before putting the mobo battery back in while power to the PSU is disconnected.