Check for a blown fuse, possibly for the interior lights, horn, or lighter. Some radios will work except for the sound when the memory circuit is dead. The memory circuit is generally tied into some other circuit that is always live. Another clue is the radio will lock up again after you turn the ignition switch off, and the station presets will reset to the factory defaults and the clock will reset to the same starting time.
If the memory circuit appears to be working properly, there might also be a remote-mounted amplifier that has a blown fuse. It is fairly common for fuses to blow when reconnecting the battery due to current surges when there really isn't any problem in the circuit. Mechanics prevent that by using a memory saver when replacing the battery. This is REAL important on many newer cars. Some of them, Volkswagens, in particular, have to be towed to the dealer after the battery is disconnected or run dead.