Long time idle? Could be BIOS settings are lost/corrupt. Disconnect data and power cables to HDD. Boot without a boot drive connected and see how it goes. May allow you to enter BIOS - check time and date - confirm settings. (Grab a copy of motherboard manual and review it - usually available at maker's website in Support). If it all works up to the point it looks for MBR, consider feeding it flash drive boot media or an OS install disk in your optical drive - just to confirm the setup is bootable.
Now focus on HDD. With system power off, attach just the power connector and start system. Does it spin? Are there strange noises? If it spins happily with almost silent whir, power down, attach data cable and try again.
Hopefully, you will see it recognize the HDD in BIOS, then the splash screen and then it tries to read MBR and it then starts the operating system.