got a technical problem - sudden lost the ability to ram preview

Basically i have a single ae file that i have been working in cc2015.2 tat sudden lost the ability to ram preview. the green bar flashes and in of says mixing audio but it basically just flashes the same frame until ( and this is the weird bit) i click on something in the menu bar such as file or edit and then it starts previewing. I've isolated most of the files so it doesn't appear to bee annoys the assets. so wondered if anyone had any idea on how stop progress?

[Moved from the forum Lounge to the After Effects forum and edited the title - moderator]