A common issue with dimmers not working properly could be attributed to the level of light the dimmer is set at when last used. Most dimmers have two components: the on/off switch and the slide dimmer (to the right of the on/off switch on the Lutron Diva Dimmers). You can check to make sure the switch portion is in the on position and that the movement of the slide dimmer adjusts the light.
In other situations, if the fixture has recently been replaced or new bulbs were recently installed, specifically if the new bulbs are LED bulbs, it’s possible the new fixture or bulbs might not be compatible with the existing dimmer. If the bulbs are not compatible, the dimmer will still operate the fixture, but it might not dim completely, or it might flicker at brighter levels, or the lights might strobe.