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.
