When I enable gridlines, x axis label is not displayed:
 
Plot[5.939663571812609`*^-15 Sqrt[
  1.9380043062246294`*^26 - 1.4020134775864955`*^30/d^2], {d, 0, 
  1000}, PlotRange -> {{86, 1000}, All}, Frame -> True, 
 GridLines -> {d2, d1}, AxesLabel -> {"d", "Angle"}]
AxesStyle -> Directive[Black, 24] doesn't work either.