Blank composition in After Effects

Solution 1.

Your computer cache may be full and AE has no more memory allocated to render new frames.

- Clear your cashe under Preferences > Media & Disk Cache by clicking the below circled 2 buttons.

- Quit After Effects and relaunch it to see if it now works.

Solution 2.

If solution 1 didn't do it, your computer memory may be too small to render in real-time your composition so what you need to do is reduce the rendering resolution of your composition in the viewport to 1/3 (third) or 1/2 (Half) the original (Full) resolution.

P.S. Don't worry this will have no effect on how the video is finally rendered to MP4. This only affects the resolution of the viewport inside AE while you're working on it.