Power PC 5400/180 stopped booting?

[adolescent]

Check the Apple library.  There's lots of information there.  Here's a PowerMac 5400 manual, but it's not a service manual, so it doesn't have repair or complete disassembly instructions.

http://download.info.apple.com/Apple_Support_Area/Manuals/desktops/0307926APM5400UM.PDF

I'd guess that the battery is pretty important.  It saves the the machines NVRAM and would probably have device information, etc. in it.