I'm with PolymerTim on this. Your machine will start up, but the error message says it cannot find the proper boot device, which should be the hard drive. The odd part is, your story says that it did boot successfully from that device the first time you started it up (after CPU re-install). So it seems unlikely (but still possible?) that you reconnected the HDD's cable the wrong way.
HOWEVER, the connection does look like the culprit. Maybe it was reconnected loosely and has shifted so it is no longer making a good connection on one or more pins. Try disconnecting, then reconnecting the cable to the HDD, both at the motherboard and the HDD ends. Look carefully at the cable. If it's a flat ribbon cable, one edge will have a red stripe along the wire. That is the #1 Pin edge. (I don't know how round cables are marked, but I am sure there is a clear identifier somewhere.) At BOTH ends of the cable, the connector must be inserted so that the red edge goes to the #1 pin on the connector. Now, many connectors have one pin's hole blocked off so you cannot do them the wrong way, but some do not and mistakes can be made. On the motherboard, look for a marking next to the connector body like "1". On the HDD case, there should be a marking, often stamped into the case metal or printed on the labels, showing which end has #1.
Even if you did not re-connect wrongly, the action of disconnecting and re-inserting the cables ends may clean off any dirt and give you a good connection. Just be sure to align things and do it carefully. You never should need to force a plug into a connector.
If that does not solve the problem, you may need to examine the BIOS Setup info to be sure it knows there is a HDD attached, and that it is set to boot from that device. Don't know about your machine, but many allow you to get to BIOS Setup by holding down the "Del" key when it first is turned on. Most will actually show you on the screen a message about this, often at the bottom, early in the boot sequence. Watch for it, in case "Del" is the wrong guess and you need to use another key.
Another less likely thing to check is the proper identification of the HDD. Most BIOS's are set to detect the HDD type automatically during boot-up, and they depend on modern HDD's to have the smarts to tell them that info. Two things can foul up that system. If your BIOS is set NOT to auto-detect, but to use preset values, it is possible the values are wrong and the system can "talk to" the HDD, but it make no sense. Setting it to auto-detect should fix this. HOWEVER, if your parent's machine was set up oddly with a HDD that actually needs specific HDD parameters entered manually to avoid whatever auto-detect might find, then you would need those parameters to enter. From your story, though, this seems very unlikely.
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