高兴得不得了。 乐得不得了。 兴奋得不得了。 幸福得不得了。 痛苦得不得了。 when you want to describe someone's feeling, you can use the combination 'adjective +得＋不得了'. 不得了 just means great in amount or degree, like 'much' or 'a lot' here. Another words, like 要死，it has the same function as 不得了，but greater. 高兴得要死 乐得要死 so on. But let's talk about it in a different way It also has the function as 不得了了，出大事了(出人命了，着火了，etc. ) 。 Like in this example, when something that is very bad or unpleasant happened, you can use the combination '不得了了, +the thing happened ' to inform someone. It's only used to express feelings like surprised, stocked, and with loud voice, because it's urgent.

it can be understood as "oh my (God)".it can be used in many situations.

it just like "very"

It used to express your superise when something happened beyond your expection.

不同的地方有不同的解释