Desktop PC powers up but gives no signal to monitor.

[chumchum]

Hello H&S
I've been away for two months and have left my desktop unplugged in the mean time. It worked fine before leaving.
I powered it up and could use it for a few minutes before getting a BSOD, I wasn't too concerned as this would often happen on the first boot and then the computer would run fine, all my windows machines were plagued with video driver woes.
After restarting however the monitor displayed "NO SIGNAL" without even showing the BIOS or motherboard splash. The keyboard and HDD activity LEDs also failed to light up. Made sure that all the connections were nice and snug on the inside and outside of the PC.
All the devices seem to be powered (all fans are running, HDD's making the usual noise).
I don't have another PC to test the individual components, I did however test the monitor with my laptop and it's working fine and so's the DVI to AVG adapter I need to use.

My computer's homemade with a Ffoxconn Flamingblade mobo, an ATI Radeon HD 5770, Pentium i7, 6GB RAM unsure about PSU but I think it's 530W.

I'd appreciate any advice.

EDIT:
I removed the mobo battery and waited a bit before reinserting it.
I now get a startup beep, keyboard LEDs light up and the motherboard splash is displayed for a while before a blackscreen error reading:
"AHCI BIOS not installed!!
CMOS checksum error- default loaded"
And then it just hangs on that.
No HDD activity on the light either.

[Taipan]

Any lightning strikes/power peaks when you where gone?

[chumchum]

I left it completely unplugged.
For some reason setting the BIOS' time seems to have allowed me to boot again.
And the newest display drivers have fixed the stability issues apparently.

I think the battery just died unusually fast (after one year)