Shell shock

In World War I, many soldiers had psychological problems. These kinds of problems are known as post-traumatic stress disorder (or PTSD) today. The term post-traumatic stress disorder is relatively new. At the time, people such as Charles Samuel Myers called this condition shell shock. Shell shock is a reaction to bombardment and fighting. Soldiers who were affected often felt helpless. Very often they panicked, were scared, and tried to flee. They were also unable to reason, sleep, walk or talk.
During the War, people did not agree on what exactly shell shock was. Some people interpreted it as a physical or psychological injury, others said it was a lack of morale. In 2021, the Department of Veterans Affairs still used the term "shell shock" to describe certain parts of PTSD. In most cases, the term is no longer used, except for very specific injuries which occurred during the war.
From World War II onwards, people were no longer diagnosed with "shell shock": The new term used was combat stress reaction: this is a similar but not identical response to the trauma of warfare and bombardment.
Modern cases of shell shock.
As outlined, the term shell shocked is used in the context of World War I, to describe early cases of what is called PTSD today. As it is a reaction to the detonation of high-impact explosives, it also has modern uses. 
When they were in Iraq and Afghanistan, approximately 380,000 U.S. troops, about 19% of those deployed, were estimated to have sustained brain injuries from explosive weapons and devices. This prompted the U.S. Defense Advanced Research Projects Agency (DARPA) to open up a $10 million study of the blast effects on the human brain. The study revealed that, while the brain remains initially intact immediately after low level blast effects, the chronic inflammation afterwards is what ultimately leads to many cases of shell shock and PTSD.