Vauxhall Astra car won't start, new alternator, lights and radio and fan work but turn the key and theres nothing doesn't even try to turn over

Tried a jump start ? Or charging battery ?

If it is since you are home from the new alternator, get the mechanic to come and finish the wiring connections. One is loose at least

Is your earth strap intact from battery to chassis and is your fan belt on okay,it,s not jumped off ,does sound like no power in battery.

tried jump start,nothing battery seems fine as all lights/radio etc all working alternater renewen 3 weeks ago was running fine after that

Is it turning over if not then starter system fault,check you are getting power to the starter solenoid,if turning over but not firing up then other issue .