Very common issue. Seems to occur more often when the vehicle has been driven then parked for less than a half hour. Then you try to start the engine and it cranks over fine but doesn't start. The fuel pump relay fails to provide power to the fuel pump due to a crack in the relay circuit board.
If you look on the relay there's a date code. If the date is older than 1991, replace it no matter what. Volvo came out with a newer relay after 1990-91 and the later relays are better made.