I will list some of the years and reasons Hajj was stopped in the past.
Corresponding to 251 AH:
Ismail bin Yusuf, known as Al-Safak, who led a rebellion against the Abbasid Caliphate, massacred pilgrims that gathered at the Arafat Mountain near Mecca, forcing the cancellation of the Hajj.
The Qarmatians were a heterodox sect based in eastern Arabia, who established their own state under Abu Taher al-Janabi.
About 8,000 people died in Hijaz ( Present day West Saudi Arabia, Jeddah, Makkah and Madina included) due to a plague. Forcing the cancellation of Hajj that year.
Cholera outbreaks in the years 1846, 1850 and 1883 AD also affected pilgrims.
The cholera outbreak coincided with the Hajj season and was severe, dead bodies piled up and deaths increased in Arafat and culminated in Mina.
An epidemic of typhoid fever and dysentery spread by pilgrims that arrived Makkah from Madina.