Hello, what to do when a grey bar loads on osx startup?

Hello,

I was restarting my iMac 3 days ago and it brought up a grey bar that loads & starts to load the bar again but only gets halfway before it shutsdown... last week I ran the disc error program & it showed Blocks on the drive were damaged and unrepairable, I was wondering if it was possible to just buy a new hd and have it replace the damaged one inside my 24 inch iMac Intel? thanks in advance... :)

Instead of having to buy a new iMac. I am disabled and live on a very low income and was hoping to maybe just be able to replace the hd even if it were used...

Do you think I would be wasting my time on doing so?

Thanks very much for your time...

best regards,

damris