I'll press space and the cursor will move and audio will play, but nothing else will happen. It won't update what I'm seeing - for example, I can create a solid layer and will show up in the timeline, but I won't be able to see it on camera.
Audio, Crash, Error or problem, Freeze or hang, Performance
We need a lot more details. If your system meets the Adobe After Effects -Minimum System Requirements, and your comp is fairly straightforward, you should have no problem previewing a few seconds of an HD comp. Without some details, you might as well have asked "Why is watermelon?"