It was attacked by the Duras Sisters, who had managed to acquire the shield modulation by bugging Geordi La Forge's visor and altered their weapons' frequency to match it. Their assault caused a warp core breach and the saucer was separated. The saucer crash-landed on Veridian 3 and was deemed unsalvageable.
See Star Trek: Generations plotline for details.
Also, the saucer section may have been salvaged eventually. If you want to argue that the "All Good Things" storyline will happen, you could assume the saucer section got fitted onto a Galaxy Dreadnought stardrive.