Having a period (menstruating). i.e. Out of action, a bit like a car in a garage. e.g. "I don't think I'll be in luck tonight lads, the missus is up on blocks".

When an automobile has its wheels removed it is not a good idea to let its entire weight rest on the delicate brake components and/or bodywork. Therefore blocks, often common house bricks are placed under the sturdier frame or axles of the vehicle.
Sometimes a vehicle is placed up on blocks to prevent wear on the tyres. This is common for expensive collector cars.