Used when you are forced to do something you didn't expect, due to sudden changes. When he broke his leg, he had to fall upon his friends for support.
Pro Tip 2/3
Financial Strain
Often refers to accessing resources like money or assets during times of difficulty. They had to fall upon their emergency fund after the flood damaged their home.
Pro Tip 3/3
Last Resort
Implies that all other options have been exhausted, and this is the final one available. With no job offers, he had to fall upon his old skills as a carpenter.