Prior to the Telecommunications Act of 1996, a "dark" station was required to surrender its broadcast license to the FCC, leaving it vulnerable to another party applying for it while its current owner was making efforts to get it back on the air.
A service can go dark for any number of reasons, including financial resources being too drained to continue effective operation of the service as being of benefit to its community of license; abandonment for a different channel or to go cable-only; complicated technical adjustments involving radio antenna repair, requiring the broadcast tower to be de-energized for the work to be done; structure fire or natural disaster that has rendered the facility inoperable; if unowned by the station, the loss of a leasehold on either the tower or the land for the transmitter, usually by sale to another party; or technical adjustments that would make it prohibitively expensive to perform the work and carry on the normal operations of the station in question.
The Great Recession was problematic for many struggling new-entrant stations, which had to move to lower channels and convert to digital television so that UHF 52-69 TV spectrum could be repacked and sold to mobile telephone companies.
If a station (silent or otherwise) must shut down its lighting system for an extended period of time (such as it being disabled by a lightning strike), it is required to notify the FAA immediately.
Under the Telecommunications Act of 1996, licensees of stations that were listed as "silent" by the FCC (at the time around 400) were warned to either power their facilities back up or their licenses would be canceled permanently.