Instead of guessing, it will be easier if you make up a MWE for a case.
But here is one case where no plot shows:
f[t_] := Sin[t]*I;
Plot[f[t], {t, 0, 10}]
But now a plot shows:
f[t_] := Sin[t];
Plot[f[t], {t, 0, 10}]
So you see why.
The possibilites are endless as to why a plot might not display at some point. To get better help please provide a MWE.
