Compaq Presario 2800 hard drive not recognized

Question

I tried to switch out the hard drive on my laptop the other day and I couldn't get it to work.  So I put in the old one again.  When I tried to turn it on I got the error ''non-system disk or disk error''.  There is no disk anywhere that would not be allowing it to boot.  I thought it might be missing files so I tried to install the operating system again since I don't need anything that's on there but when it gets to the part where it asks me where to install it, there is no hard drive.  So it doesn't recognize it although it is plugged in snugly.  Please help me some one!  Thank you!