Taking a neutral wire from another meter connection?
TL;DR: CALL YOUR ELECTRIC UTILITY ON THEIR EMERGENCY NUMBER

It sounds like you live in a duplex with dual services -- in that case, call your utility ASAP.  What the electrician did was a very dirty hack that will not pass Code muster, and likely screw with the electrical metering -- it will also break in interesting ways if later on the utility puts the two units on different transformer secondaries.

BTW -- an open neutral is an emergency situation, as it can easily be a fire and/or shock hazard.