In an empty new notebook

Reduce[l1==1+(b*x*wpe*td)/l1,l1]

tells me that (l1==(1-Sqrt[1+4 b td wpe x])/2 or l1==(1+Sqrt[1+4 b td wpe x])/2) and l1!=0

Is one of those solutions sufficient for you? Or do you need both of those?

If I begin your notebook with

l1=(1-Sqrt[1+4 b td wpe x])/2;

and remove your definition of l1 then I get an empty plot.

If I insert before your Plot

Table[a31,{x,1,5}]

then I get

{8.55988*^-9/Abs[s], 1.09454*^-9/Abs[s], 3.27572*^-10/Abs[s],1.39018*^-10/Abs[s], 7.14658*^-11/Abs[s]}

and I see that s has not been assigned a value in your notebook.

If I assign s=1 at the beginning of your notebook then with these changes I see a plot.

Please test all this very carefully to make sure I have not made any mistakes.