First check your INPUT settings in the Project Settings. And make sure MoveForward and such has the right scales. W should have a scale of 1.0 and S should have a scale of -1.0.
After that in your character BP, make sure when you set the InputAxis MoveForward node, connect it to the Add Movement INput node and world direction should be connected to Get Actor Forward Vector. Like such:
Other than that, try to close the project down and re-launch it. I’ve had some weird issues lately where something was working then all a sudden stopped, so I had to relaunch the project and it worked fine again. That’s with the latest 4.16.2 engine.