My computer won't boot up

Hello,

I have an iMac and bought Parallels Desktop so I could play PC games on my iMac. I didn't realize that my iMac could not handle this much memory. The games worked to a degree but I then stopped playing them for a while then tried playing one again and it crashed. I tried restarting but my computer wouldn't boot no further than the spinning wheel before you see the blue screen. It was even more strange that a loading bar displayed underneath the the spinning wheel and would load a third of the way, then disappears. I then did an applejack and that worked fine. So I tried uninstalling Parallels and restarted the machine. It then refused to boot up again, so I tried an applejack (numerous times) but every time it failed to boot up.

I was thinking about replacing the Hard Drive with a new one because it is only a 500GB hard drive and would like to have a 1TB hard drive instead, and put my old hard drive into an external hard drive case to retrieve my files. Do you think that replacing the hard drive would fix my issue?

If so can you please tell me how much it would cost to replace it or if not can you please how much it will cost to repair my problem.

Kind Regards

Jacob Lindsay