了 (le) has a lot of uses. You probably first learned 了 as a particle that tells you an action is completed, which is also known as 了1. This article is not about that use of 了; it's about indicating a change of state (了2). In other words, there is now a new situation, or there is about to be a new situation. This whole "change of state" idea can take numerous forms, so it's helpful to view them individually, alongside some English translations.

General Change of State 
When used in this way, 了 is placed at the end of the sentence to mark that the whole statement describes a new situation.
Examples: 

Expressing "Now"
The word "now" doesn't always need to be translated as 现在. You'll notice that in many common expressions, 了 is used in place of the explicit word for "now."

Expressing "Already"
You can expect to see the word 已经 (meaning "already") in these sentences, which frequently pairs up with 了, but note that sometimes that feeling of "already" can also be expressed with 了 alone.

Expressing "Not Anymore"
In a negative sentence, the sentence-final 了 can taken on the meaning of "(not) anymore" or "no longer."