Toddler (undiagnosed)

Hi, 

I'm a Dad to a 23mnth old girl and 11wk old boy,  

Our little girl has showed signs of "switching off" for a little while but over the past couple weeks she's started having "meltdowns" and nothing will calm her till she's exhausted herself, is there any ways of being able to try breaking the cycle or is it just something that can't be helped, and has to stay the coarse till its over? 

Just overwhelmed with it and don't know where to look for any answers.

Thanks Dan