there's a chance that the motherboard (or the RAM)is in trouble. Do you get any POST (Power On Self Test) information at all when you try to boot? If you don't, something is fundamentally wrong. There are some things you can try:-
1. Replace the CMOS battery - they're easy to obtain from any computer shop, and sometimes jewellers have them as well.
2. Carefully check every single connector on the motherboard - it's surprising how easy it is to overlook a loose cable.
When you say that you replaced "the data cable" what do you mean exactly - which cable was it?