What's up with my computer?

Since the "can not find hard drive" message appears with both hard disks, they are unlikely to be the problem.  Assuming you have SATA hardware, check the SATA settings in BIOS for changes (from IDE emulation to AHCI or vice versa).

This problem is typically due to a failed data cable, or more rarely a bad power cable, so you should try a cable swap (after reinstalling the old hard disk, of course).

If possible, check the voltages on the power plug to rule out a power supply problem.

Hard disk controllers occasionally fail.  In BIOS, check whether the hard disk is detected.  If not, power down and reconnect to an alternate port, then power up and recheck BIOS.  A single controller may drive several ports that could all be dead, but multiple controllers may be onboard, so be sure to try all the connectors.

If it's a SATA hard disk and you have a SATA CD or DVD that's detected in BIOS, then at least that controller and port are active, and you can try the hard disk on that connector.

If you just installed the new hard drive and did not reload Windows that explains why it will not load.

The new disk is blank. You must install Windows and other software from your backup.

Ensure data and power cables are connected well at all ends

Ensure the HD is spinning up when the PC is turned on

Try another SATA port on the motherboard - you may have to enable it in BIOS

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