As Bud said. The wheels are needed for most land-based vehicles to be useful. "The wheels fell off" means something fundamental happened that ruined it, some plan, something. . That's life. You have a job. You have an apartment, You don't loathe your coworkers. You are going along fine, then out of the blue, the wheels fall off.

In the most literal sense, it is a vehicle whose forward motion is dependent on wheels. When the wheels fall off, it ceases to move forward. This can be used to describe other aspects of the vehicle. "I'm not going to buy a new car, I'm going to run this one until the wheels fall off." This just means I'm going to drive it until it no longer works. You can also use it to talk about anything that can eventually fail. "We had a great relationship for many years, but then the wheels fell off."

Can you put this into a sentence as it depends on the context?

The only way I have heard this used is "the wheels on the bus have finally fallen off". It means someone is in a bad situation, and the situation is now very bad. Often used to infer someone is going crazy.