|Very long shot...|
Download and burn an ISO of Ubuntu to a dvd. Then see if it will boot with that dvd.
If it does... then the motherboard is ok.
If it does boot up that way, then see if you can access the hard drive. If you can... copy all critical files (personal stuff) to to external storage (typically dvd, but another hard drive too is nice). Then set about discovering what's really amiss.
The Linux boot routine effectively tests out a system more or less completely; as it loads itself into RAM only. The local hard drive is not involved at all, unless you opt to install Linus - which do NOT.
Ubuntu.com is where to find the free ISO.
If you decide tp replace the system entirely, remove the local drive and via a usb adapter or similar, you can access the contents. Typically one either puts the drive into a suitable enclosure, or uses a dock, or a cable set; the external enclosure is of course generally the best. The drive is then safe and can be used accordingly. And again save anything critical on it to dvd as well.
Do not pass the system with the drive installed; or if you do then first recover files - usb adapter etc., - then reformat the drive. But check the dvd copies of data are accessible on at least one other working system.