Negative sentences with a change-of-statele了 at the end show that a situation or action has stopped or a plan has changed. “bù不...le了” or “méi yǒu没有 ... le了” means “no longer ...” or “not ... any more”.
Xiànzài shì chūntiān le, tiānqi bù lěng le. 现在是春天了，天气不冷了。 It's spring now, (and) the weather is not cold any longer.
This sentence shows that the cold weather has come to an end.
Tā bú zài zhèr gōngzuò le. 他不在这儿工作了。
He does not work here any more.
This sentence shows that 'he' has stopped working here.
Míngtiān wǒmen bú qù Běihǎi Gōngyuán le. 明天我们不去北海公园了。
Tomorrow we are no longer going to Beihai Park .
The sentence implies that originally 'we' planned to go to Beihai Park but now the plan has changed.