Check all the subframe mounting bolts from under the car. The inner ends of the threads can get rusted so they'll seize half way out and may well shear off if forced. If someone has had a go, they might have left a bolt seized part way out - causing your problem. A "tight" bolt with loose washer with be a clue.
To much force will shear off the bolt. Seized or sheared-off, it will need an access hole(s) cutting into the floor to clean the protruding threads or to drill them out and weld new nuts onto the back side.