If it is the city, it may be because of a sewer leak that has to be repaired by Public Works or it may be Waterworks fixing a water main. This type work is done in “off hours” due to urgency – it may affect many residents or may be posing a health or safety concern. Please note that Virginia Natural Gas repairs gas line leaks and Dominion Power repairs power lines that are down.
Your first step should be to go out to see what color truck it is and/or see what words are on the truck. Waterworks trucks are blue. Virginia Natural Gas and Dominion Power both have their name on their trucks.