It's possible Element has crashed as it does require your graphics card. Make sure everything is saved, shut down AE and possibly restart your computer and it should come back. You might need to update your graphics card drivers but it is also worth checking your image sizes. Element might not be able to handle very large images
Without any info about your system, your graphics hardware, your settings in E3D, what models and textures you use and so on nobody can tell you much. As the cactus was already hinting at, E3D is a major GPU resource hog and you could simply run out of juice just like there could be buggy drivers and conflicts with AE's own hardware acceleration stuff. Impossible to know without any exact info at hand.