DON"T PANICFirst thing is to try and recover your computer. It is more likely that something other than your mother board has failed, but even then you have better options than starting setup from scratch with a new PC.
- To start with, check RAM (reseat, restart)
- No beeps from PC (warning, diagnostics)? Of course, maybe your PC doesn't have an internal speaker.
- no CPU fan trying to start?
- try replacing the CMOS battery
- try removing and reseating everything else (RAM, GPU, drives). See if PC will try to boot in basic configuration (nothing), then with RAM (one stick at a time), and then progressively as things are plugged in.
- Your symptoms suggest power supply more than anything else, so get another power supply (you said you have a spare PC somewhere?) and try that, or grab a new one. It is not unusual for a PC to want a new PSU occasionally, especially if it is run continuously and/or in hot/humid conditions.
I think it is unlikely your drive has failed, because then the PC would still be trying to boot and producing an error on startup. If it has failed with a weird nasty short or something that upsets your motherboard, then unplug it and the PC should try to boot. You can test the HDD/SDD and data separately anyway, then make a backup and port that setup over to a new computer (or new motherboard, if that is what has failed). If your drive really is dead, then go to recovery from backup. No backup? Well.....
On the other hand, a failing HDD could be explain why your setup kept taking longer and longer to startup (taking longer to read critical data), which you were reporting before. But then the motherboard should be throwing errors - and slow startup could also be explained by a struggling PSU.
If all that fails, phone a friend with better IT skills

Good luck!