The MAF iteself should not be the problem, but changing it has probably upset the ECU. Presumably your mechanic didn't plug it into any diganostics? It may be as simple as having forgotten its target idle speed. This happens occasionally, normally with the result that that the car starts and dies straight away. A soft reset can be performed by turning the ignition on so that all the idiot lights come on but not starting the car. Count to 90 then start (without tuning it off first). If that fails try disconecting the battery for half an hour, making sure you have the code for the radio and your remote key fob works before you do. If these fail, you're probably going to have to plug it into the diagnostics.
GTV and Racing Driver