死了 literally means “have died/died”, but it has a very interesting use that has nothing to do with “die”, or in other words, it is a figurative use.
Formula of 死了
an adjective + 死了= so/very/really/extremely + adjective
e.g. 累 tired + 死了 = so/very/really/extremely tired; 高兴 happy + 死了 = so/very/really/extremely happy
When they told me I’d won a car, I absolutely freaked, ie was extremely happy. 听说我赢得一辆小汽车, 简直乐死了.
I’m not accustomed to staying out so late and I am extremely tired. 我不习惯在外面呆得太晚，我累死了。
I find Russian grammar very difficult. I find Russian grammar very difficult. 我发现俄语语法难死了。
It’s terribly noisy living near the airport; planes are coming over all the time. 住在机场附近吵死了，飞机总是从上空不断通过。
Though extremely tired after a whole day’s journey, they still pressed on. Though extremely tired after a whole day’s journey, they still pressed on.尽管走了一天快累死了，他们仍继续前进。
“死了” and “不得了” have the same meaning regarding this special formula. What is also worth noticing is that both formulas of “死了” and “不得了” are mostly used in informal context.