Won't boot up on start up.

That’s a telltale sign of hard drive failure, especially on these HPs. When the drive is half dead, they almost always fail like this and a fully dead drive throws an actual POST error. Refer to this guide up to Step 4 and don’t remove all of the screws - just the ones for the drives.

Some of these are dual hard drive configurations (look for 2 hard drive cables to confirm this), so there is also a chance a secondary drive you can live without (keep the mounting hardware just in case!) will bring the system back, especially if it means your data is safe. See if you can screw the caddy in business as usual, leave the cable connected and keep the drive screws in a safe place you will remember. Worst case scenario you buy another set on eBay if that’s all you lost.

HOWEVER, when one drive dies on these dual HD HPs you need to be careful because the surviving drive from the pair usually dies next - but sometimes you get lucky and it doesn’t. You can chance it, but take regular backups just in case. I’m not fond of this idea, especially on HP Consumer models. Almost nobody knows that one drive knocks the system out and doesn’t understand the pair is on the way out. Because of the high risk, your best long term option is to get a single SSD (500/512GB minimum) and get away from the dual drive nightmare if yours is plagued with this configuration. I get it on a business model like the Elite series or HP Z (the buyers understand the risk), but no end user ever gets it until one of the two drives quits and creates an issue like this.