Why does the $\LaTeX$ cases environment not work?

Question

In the last few days I have twice tried to use \cases in $\LaTeX$ and gotten nowhere. Why does it not work?

Or, if it does, how does it work?

Answer 1

Care to share an example, below taken from here appears to be displaying correctly for me here on meta.

\begin{equation*} |x|= \begin{cases} x & \text{if $x≥0$,} \\ -x &\text{if $x\le 0$.} \end{cases} \end{equation*}

Code used to generate this:

\begin{equation*}
    |x|= 
    \begin{cases} x & \text{if $x≥0$,}
    \\
    -x &\text{if $x\le 0$.}
    \end{cases}
    \end{equation*}