The hard drive is the first point of failure I would expect. How did it fail? Do any lights turn on? Does it make any clicking or scraping/scratching sounds? Does it post a boot screen or a failure code or just a blank screen?
Without more information, the easiest choice is buy a new DVR and swap it out.
Do you want another 8 camera system or do you want more/less camera support?
Do you want a higher resolution system? If so you'll end up needing to replace your cameras too.
Are your current cameras analog or digital? Analog taps out around 720p, but you won't have to replace all your cabling if that is what you're already wired for. 960H is the format name.
Digital system will get higher resolution, cost more, require you to replace your analog lines with ethernet cabling.
When my old analog system died I replaced it with a 960H dvr and cameras because I didn't want to crawl around in the attic over again and run ethernet cabling. The resolution difference from cheap ~400TVL to ~800TVL cameras was a big difference.
I ended up with basically this 16ch Qsee DVR but different cameras, bought separately, for < $400 all off amazon.
https://www.amazon.com/gp/product/B00IKR21W0/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
Been happy with it so far. The bullet cameras with built in IR all have the issue that spiders will build their web right over the camera if you don't provide alternate IR sources.