how did the northern song dynasty get overthrown?

The government did not have enough money to organize the country.
The government system could not adjust to the new society.
The army is so week that could not defense the invaders.

It got overthrown or fell when the Yaun court attacked the Southern Song

It was conquered by the Jurchens, a people similar to mongols but from Manchuria, who formed the Jin dynasty. This dynasty was actually extremely wealthy, one of the richest in China, but disdained the military and instead spent their money on Taoist temples and palace luxuries.