Why am I getting no power when I pedal?
This sounds like a issue in your freehub/freewheel (both are mechanically the same for this question).  This is what allows you to coast without pedaling.  Inside these bodies are a set of "pawls" which will flip down and engage a series of splines when rotated "forward", and propels the bike forward.  Then, if you move them in the other direction "backwards", the pawls are designed to be pushed out of the way of the spline.  Given how old your bike is, there is probably a build up of dirt and drying out of grease inside these that are causing the pawls to be stuck in the "up" position and not able to grab the splines.  In this case, your local shop should be able to fix that right up by replacing the part.