What would you do if a girl got mad at you for no reason and before she used to let you be with her almost every second of every day?

If you can't really think of something you did to make her angry; ask her directly why she is mad at you now.

a girl got mad at you for no reason...

guess not....maybe you did something wrong to her that you just forgot or you never recalled....go talk to her.....one on one....

come on it's simple, she likes you!

I would figure out what it was that got her mad at me, most likely by talking to her and figuring out what happened.
Then, I would try to make amends if I screwed something up.

So basically, figure out what the problem is before you try to make things better.

Well, it depends. What was your relationship with her at that time? But here are some possible choices:

a) you did something but your just not aware of it (you need to ask her directly what happened)

b) she likes you and got mad becuase your not taking the initiative (you should suprise her with something romantic)

c) outside stuff like family or other friends are just pulling on her stress strings and she's lashing out becuase she's frustrated (you should back off while letting her know that your there for her if she needs anything and actually go through with it if she does ask you for something).

d) if you guys are dating and she's doing that then you should definitely ask her what's wrong and be completely open to what she's saying. Odds are you won't like it or she would've brought it up before.