I seem to have an issue with the spacebar in AE CC.
When I press the spacebar the current comp is not play, rather AE switches to another comp and plays that one instead.
This is an intermittent problem and the only way to fix it is to shutdown AE and reboot the software.
Any ideas ?
Check to see if the comp it switches to has the "lock"key on.
Do you have Always Preview This View enabled for the composition that plays?
In addition to what others have said which should answer your actual question, spacebar is not the normal way to preview things in AE. You should probably go through these resources: Getting started with After Effects If you're doing something as fundamental as previewing in an unusual (and mostly wrong) way, I can only imagine how the resources linked there will help you avoid a lot of frustration in the future.