Hello,Welcome and thanks for your question. It wasn't there one day and disused the next.
The Forum dated from at least 800 BC and was originally at sea level and very small (not much larger than a US football field). In the early years it suffered greatly from floods, and as a consequence sewers were built. As centuries passed and buildings grew up around it, the open area became even smaller. By 200 BC another larger foum had been built. The last of the larger forums was constructed about 500 AD.
The reason for the gradual disuse of the oldest forum was simply that they outgrew it as the population increased over a period of 1,500+ years.
All the best,
Wow. We can answer one thing per question. A history of the forum is beyond the scope of this site. I am going to opt out since I have already provided the information required by the original question.
Here is an excellent old book, long out of print:
Here is another you can read online:
Watkin, David (2009). The Roman Forum. Harvard University Press. ISBN-13: 9780674033412. If this one isn't in your local library, you can order it from Barnes and Noble.
You'er so very welcome. Enjoy your reading!