You will have to provide a minimal working example (minimal example that still has your error). If the example is too minimal, the error might not be apparent. For example, compiling just what you provided I am getting the expected output:

I just added a documentclass, began and ended the document.
\documentclass{article}
\begin{document}
\rm \setlength{\parindent}{1cm} Estimatorul nonparametric pentru functia de distributie cumulative, $F(x)$, in punctul $x$ este raportul de puncte intermediare care sunt mai mici sau egale cu $x$.
\begin{equation}
\mathop{F}\limits^{\wedge } (x)=\frac{\# \{ x^{t} \le x\} }{N}
\end{equation}
\end{document}
There could be something that you didn't provide in your question causing this behavior. Find what it is by reducing your example until the error dissapears, and then expanding it again until the error becomes apparent again. You might pinpoint the troublesome spot yourself that way, or at least narrow your problematic area of code.