沉住气

bù yào huāng qiān wàn yào chén zhù qì

不要慌，千万要沉住气。

Don't panic. You must keep calm.

bù néng chén zhù qì

不能沉住气

lose one's composure

shì dào lín tóu yào chén zhù qì

事到临头，要沉住气。

Now that the situation becomes critical, one must keep calm.