Well contrary to a lot of opinion the radio does n't make the aeroplane fly. So presuming a conventional aircraft they are all designed (and certified) to fly without electrical power. It might be inconvenient but they do still fly.
If you are outside controlled airspace, stay outside it and divert to a field that accepts non-radio traffic. In it, continue to you destination and hold until you get light signals (could be forever!!)
If you're really unlucky and in the clag - go find some low ground or the sea to descend into. This one is really a little stressful!!
But the chances of a sudden electrical failure are very low - it has happened, usually with a short circuit that might cause smoke or a fire. But electrical problems generally give a lot of notice - most aircraft will run on their batteries only for much longer than on the fuel in their tanks!