If Pro Tools isn't playing audio, first check your monitor signal path. Pro Tools connects with your computer via a hardware interface, such as an Mbox. This interface must be connected to a set of monitors for proper playback to occur, because Pro Tools will not play through your computer's internal speakers. Ensure that these audio monitors are connected and powered on and that the interface's volume control is turned halfway up. Some interfaces include a monitor mix knob, which must be set to "playback."
Pro Tools features multiple signal paths, making it easy to misroute the intended playback. Most Pro Tools sessions utilize the "1-2" output bus as the default audio output routing. Ensure that the tracks you'd like to hear are routed to this "1-2" output bus via the routing selector within Pro Tool's edit window or the routing matrix selector in the program's mix window.
Pro Tools also features the ability to manipulate the audio interface's routing scheme, which can easily misroute audio playback. Select "Hardware preferences" from the menu located at the top of the program and reset all interfaces to "default" to restore standard audio routing.
Each channel and track within Pro Tool's mix- and edit windows feature both mute and solo buttons. Engaging mute will stop audio playback for the selected track, whereas solo will typically play only that track while temporarily muting all others. Ensure the intended audio tracks for playback are not muted nor soloed.
Each track and channel within Pro Tools also features a volume control. Ensure that volume automation has been disengaged by clicking on the "auto" button, then check that a healthy volume has been set. When playing back, the volume meter located next to the channel slider in the mix window will indicate proper playback.