Lovecraft describe Innsmouth as once thriving town notable for it's shipbuilding, fishing and worldwide trade connections with mentioned industrialization efforts. But since early XIX century town economy collapse due to War of 1812, overfishing and industry moving out or being closed. Early XIX century was a bad period for New England economy and some small towns do slowly vanish in this period similar to fictional Innsmouth? If so, what it caused and it was efforts to stop ongoing decay in region.
I certainly hope you can get to see Newburyport sooner or later, for its antiquity & desolation make it one of the most spectrally fascinating spots I have ever seen. It has started me off on a new story idea—not very novel in relation to other things of mine, but born of the imaginative overtones of such a place. My scene will not be called Newburyport, but will be the accursed sea-town of Innsmouth, between Newburyport & Arkham.
Lovecraft had a tendency to base some of his fictional Massachusetts towns on real places - Salem was the basis for Arkham, Marblehead was the basis for Kingsport, features of Athol were incorporated into Dunwich, and Newburyport became the basis for Innsmouth - although Newburyport was never so fallen and decayed as Innsmouth. Lovecraft would, with his limited means, travel far in Massachusetts seeking antiquarian interests - old houses and buildings, rural farms, that kind of thing. In one postcard Lovecraft wrote:
Working slowly homeward via the quaintest towns I can find. Portsmouth yesterday, Newburyport today. It will cripple me financially, but it’s worth it—for this quaintness & antiquity form the real fuel & pabulum of my imagination. Newburyport is exactly as it was in colonial times—a study in colourful stagnation. At sunset today a marvellous silver mist rolled in from the sea, giving everything an antique & ethereal glamour.
Newburyport was split off from Newbury in 1764, and most of the economy was based around being a port, and activities like whaling and the Transatlantic Slave Trade helped fuel the bustling economy. After the Civil War, both declined - although Newburyport was never in the dilapidated state that Innsmouth was depicted as, it was in decline as shipbuilding and fishing became less prominent, businesses fled, and population even declined a bit between 1920 and 1930, according to census data. Currier's History of Newburyport Mass, 1764-1909 can give you some more historical details, if you're interested.
Newburyport is still there; some of the older sections have been razed and rebuilt, notably the downtown, and the local economy has shifted to focus on other things than fishing. Versatility, rather than specialization, tends to be a hallmark of towns and cities that persist for long periods of time. Not every town was so lucky; Massachusetts has its ghost towns like Whitewash Village, whose harbor was destroyed by a hurricane c. 1860, or the abandoned mining village of Davis. In most of these cases, the populations were small and most of the people derived their livelihood from a single industry. Newburyport was never that insular or narrow in its focus.