Nobody can know. You have not provided any system information or details about your project, your comps, footage and effects used and whatnot.
This error can come up, when your work area in- and out are set on the same frame in your comp.
is this the case? expand your work area (hit B for in and N for out) for preview.
if this is not the case, as Mylenium said, we need more information about your Project and system.