Internal HD Not Recognized?

Question

In March, I bought a secondhand Macbook Pro mid-2010 from a friend (serial #73******ATN). It was working fine until this past week when I was using Gmail on Chrome and my browser froze. So I switched to a different application and a few seconds later, everything froze up and I got the lovely spinning pinwheel. So after waiting a while, I powered it off and restarted and then it was at the screen with the apple icon for a long long time, just loading and loading. So I tried to reboot a few times and it would just not get past the apple icon to the login screen.
I set it aside for a while and then tried the same thing (this was at home where I have a recognized network). Then instead of going to the loading apple icon screen, it told me to choose a network. My home network was not listed so I typed the name/pw in manually and it rejected it. I did this several times and I know from the past that what I typed was correct, so I do not know why it got rejected.
So then I powered it off for the night. I started it up in the morning and got the connect to network thing; powered off; took it to school (different network), started it. This time, it did not tell me to connect to the network. I got the infinitely loading apple icon. So I closed the lid (it did not go into hibernate as it should); left it there. I came back three hours later to find the battery had drained. When I plugged it in and restarted, all of a sudden, it worked again. I logged on and used it without problems for the next four hours. Took it home, and about one hour into my time at home, Gmail crashed again and then the rest of the apps crashed and ever since that, I have not been able to restart it. I have re-tried the whole draining-the-battery thing, to no avail.
Thinking it might be a corrupted HD, my friend and I bought a new HD, replaced the old one, stuck in the installation DVD, and were able to open the diagnostic thing. Lo and behold, the HD just won't show up. The only item visible was the installation DVD.
So we took out the new HD, put the old one in, and still the old one would not show up. So now we think it's something else like a cable. The best thing would probably be to go to the Apple Store, right? But I do not have Apple Care so would they still run a free diagnosis? Any other suggestions what to do? And why would it tell me to connect to network when at home, but not when at school?
P.S. I am fairly computer illiterate so I would appreciate breakdown of technical terms. Thanks a lot!
<Edited by Host>

Answer 1

The seller should have also gave you the system discs that originally came with the computer per Apple's license agreement.  If not, you need to get them from him.  You need them to troubleshoot.  You can also get the system discs from Apple.
Replacement System Discs can be obtained from Apple's Customer Support - US [phone] for a nominal fee. You'll need to have the model and serial number of your Mac available. 
If you're not in the US, you may need to go through the regional Apple Store that serves your location to find the contact number - http://store.apple.com/Catalog/US/Images/intlstoreroutingpage.html
After you get the discs do the following -
Disconnect all peripherals from your computer.
Boot from your install disc & run Repair Disk from the utility menu. To use the Install Mac OS X disc, insert the disc, and restart your computer while holding down the C key as it starts up.
Select your language.
Once on the desktop, select Utility in the menu bar.
Select Disk Utility.
Select the disk or volume in the list of disks and volumes, and then click First Aid.
Click Repair Disk.
Restart your computer when done.
If you still cannot see your hard drive, you will need to purchase DiskWarrior which is known to find & see the hard drive.
Repair permissions after you reach the desktop-http://support.apple.com/kb/HT2963 and restart your computer.