Well the first thing I do is wait. I know its getting dark but that fish still needs to settle in(most of the time). This has happened to me too and most of the time if you cast to soon you'll just spook it. I try to see if the fish will start taking naturals. The fly I would put on is gonna be an unweighted tellico or tennessee twenty incher. I want a fly that will sink but not to fast. If you hang up you'll really alert the fish. If it reacts to my nymph but doesn't eat I'll try one more drift just to see if its the fly or my presentation. Then I'll change to a fly that I have confidence in and restart the process. I'm assuming your hidden from the trout because if he can see you it's pretty much over.