I would say:
1. Remove all expansions, harddisks and floppy drives
2. Reseat all socketed chips
3. Connect an RGB (15kHz) monitor and make sure the monitor and cable is known to be working
4. Switch the A2000 on and the "Insert disk" screen should show
If it does not, swap each of the socketed chips (from an A500 for instance?) and check after each change.
I doubt that a service manual will be of much help unless you are a technician and able to trace (and repair) faults in the motherboard itself.