Bit of a vague problem report - need more detail.
Checks:
Are all the plug leads actually on correctly.
Are the coil plugs on the correct coils.
Crank sensor fitted, unbroken and plugged in.
All disturbed electrical connections checked.
Battery positive cable properly connected to starter.
Battery negative cable properly connected to gearbox and car body.
Battery fully charged, not just left for the three weeks since first post.
Details:
When key turned to 'on',
Do battery and oil warning lamps light.
Does immobiliser light come on, and go out.
Does fuel pump run for a few seconds.
When key turned to 'start',
Does engine turn over.
Does immobiliser light illuminate, steady or flashing.
Does oil light go out.
Does it try to fire - occasional coughs.
Does it spin easily - too easily, or sound similar to the old one.
Does the clock reset to zero, or flash, or both.
Others may add more to this.