Which effect of a lethal dose exposure of X-rays to the body?

By definition a lethal dose of x-rays would cause death.

Prior to death, the person would show signs of radiation poisoning including nausea, vomiting, hair loss, bloody vomiting (hemoptysis), bloody diarrhea (hematochezia), dizziness, low blood pressure (hypotension), slow wound healing, uncoordinated movement (ataxia), and seizures.

In practical purposes, a lethal dose of x-rays is nearly impossible since modern x-rays have such low levels of radiation.

For instance a single x-ray view of the chest is only 0.05mSv, which is the same amount of radiation you absorb from the ambient environment in 5 days. So, your risk of developing cancer, is only increased as much as it were to be if you were 5 days older than your real age.

For reference a lethal dose of radiation is approximately 30 Sv. So to get a lethal dose of radiation through x-rays you would need to get more than 600 x-rays in one sitting.