Turn all of the lights up on the tower. Do this by moving all sliders controlling lights on that tower to the top of the board. Check to see if one or more lights may be malfunctioning, or if the entire tower is affected. If it is one or more lights, and not the whole tower, proceed with checking each fixture. If it's the whole tower, proceed with checking your dimmer.
Trace the power cord of each non-working fixture back to the dimmer and make certain all connections are firm. If so, unplug the power cord from the dimmer and plug it into an extension cord from a known working source. If this solves the problem, proceed with checking your dimmer; if not, continue checking the fixture.
Replace the bulb. Unplug the power cord and turn the power switch off, if so equipped. Replacement varies greatly from fixture to fixture. Consult your owner's manual to find access to the bulb. In general, wear rubber gloves to keep your skin oils off the bulb. Remove the lens, or access panel.
The bulbs come in three basic types. Twist-in Edison-style bulbs turn counterclockwise to loosen, like standard household lights: Replace these as you would a standard bulb.
Press and turn fixtures work similar to automobile tail lights: Press in on the bulb -- turn it a ½-turn to release, and pull it out.
Plug in bulbs -- such as halogens -- pull straight out and plug in the same way.
Plug the light back in, and check to ensure that any power switches are in the "On" position. If the problem persists, have the fixture professionally serviced.
Inspect the dimmer. There should be a red power light glowing -- to show that the unit is on -- and a green light glowing, to show power from the board. Check power cord connections and power switches, if no lights are showing. If this does not solve the problem, continue on and replace the fuses.
Check to be sure the DMX cable from the light control board is connected, if the green light is not on. This is a three prong, microphone cable-style connector that plugs into the dimmer. Reconnect this, if needed. Trace the DMX cable back to the light control board -- check the connections there -- and with the other dimmer packs. Consult light control board manuals for troubleshooting tips -- if this does not help.
Locate fuses on the side of the dimmer. These are typically housed in small round, twist-out holders. Turn the cap of the holder counterclockwise to remove it. If the fuse is cloudy or black, or the glass or metal strip are not intact, replace it with the same size and amperage of fuse. If this does not solve the problem, consult a professional.