First, check to make sure that you haven't fouled up the positions of the layers by changing the rendering option to Classic.
If it doesn't work there select your text layer and press the U key twice to reveal all modified properties of the text layer and start resetting things until you get the text animator to do the things you want it to do.
Second, if your machine does not have a NVIDIA CUDA card that a is working for Ray-traced rendering GPU acceleration reconsider using extruded text. If you want to continue then turn on Ray-traced rendering for the text layer and work with the rendering settings in the advanced section of the Comp Settings to get the image you want.
A better option if you are using the latest releases of AE would be to not use Ray-traced rendering but instead select your comp (hopefully with a camera) in the Project Panel and then Export a C4D file of the animation. Now import the C4D file and use C4D lite to extrude your animated text. You will have more options and it will render much faster. The only downside is that you will have to learn how to do some basic things in C4D and you won't need your text layer anymore.
Many thanks for your reply!
The rending option is still set to Ray traced so that's not causing the problem - it seems to stem from when I enable the per character 3D option when animating text. When I turn this off, the text reappears, but unfortunately any animations I've done in z-sapce also disappear.
My Mac does not have the NVIDIA card required. I have seen in a couple of training books that you can choose a cinema 4D render engine, along with the classic and ray traced ones, directly in after effects without having to go into the main Cinema4D programme. Has this option now been removed, as this would solve (possibly) quite a few issues!
The reason I mention this is that frustratingly Cinema4D will not accept my text layers when I import.
First, make sure your apps are completely up-to-date.
Second, try switching to the classic render and see if your text appears. If it does not then you have filed something up when animating the text layer. The easiest way to figure that out is to view the modified properties of every layer that could be causing a problem including the camera.
If you are using any 3-D layers in a composition you should always add a camera.