Hard drive and screen

Yesterday my computer worked fine, but today when i first turned it on, i got a hard drive failure, so i turned it off and back on and it was just testing memory for ages, and wouldn't stop - it went over 1000000k - but i don't know what that means, and i still got a hard drive failure, then i restarted it one last time and not the screen wont come off of standby, so im not sure what to look at first - if i buy a new hard drive will the screen come on? if not how will i be able to install a operating system? the ram because the screen not coming on? but then if was on earlier, my graphics card? i tried another and it still didn't come on, tried another monitor and still nothing.

If anyone could give me a idea of where to start first it would be very helpful, also i tried to reset the bios, just to see if it helped and it didnt