In general:
1.if can crank,check to see if fuel arrive to the engine like one forumner said.if fuel does not arrive to the engine then check the internal fusebox wiring and all the fuse.most of the time the fuse and relay is position in the engine bay but its worth checking
check for spark,if there is none then check the coil,wires and everything relating to the ignition.
if fuel do arrive and spark is present to the engine bay,plug in a diagnostic computer and make sure the ecu is in good health and all sensor reading and setting are in order.if there is an error then trace the error as i bet some critical wiring are installed incorrectly.
2.if u cannot crank:
check the main key console wiring and make sure power is at the wire when it should be.if the mech made an error here than whack him with an engine support tool.this is the heaviest steel one man can pick up at the workshop.
if that is fine then check the wiring on the gear console area.if its an auto the there is a switch which detects the gear in N or P position to allow the car to start.some car have this switch outside but some have it inside.double check the wiring/switch,make sure its functioning.on mercedes,if the switch got problem then either the engine can crank but cant start or reverse light wont turn on.if yours is a manual,oso check the gear console area,some manual car oso got the switch.
check your immobiliser be it built in or aftermarket.
worse case is that the mech did not disconmect the battery when doing the job and a shorted wire fried the ecu.