There are a few things that can happen when a gas line is shut off for maintenance, that might cause a pilot not to relight when the gas is turned back on.
Air in the line
If the pipe is cut; and/or replaced, there can be air in the line. To remedy this, you'll simply have to bleed off the air by opening a valve until all the air is released.
Junk in the line
Sometimes during maintenance, small particles can become dislodged. These particles can be carried by the gas down the line. Because the orifices tend to be very small, they can easily become blocked by this debris. To fix this you'll have to remove, and clean the orifices. A drip leg near each outlet will diminish the chances of this occurring, but may not completely eliminate the possibility.
When working on gas lines, valves at each fixture/outlet may be closed along with the main valve. In this case, you'll have to locate each valve and open it.