Could be anything from a plug-in effect flaking out due to technical or licensing issues to something systemic interrupting everything. since you didn't provide any details about your settings or system, nobody can even begin to guess...
> Oh, and yes, I do have the entire time-line selected.<
You may have chosen a single frame render setting. Your output module or render may e set to work area nad you have somehow set a beginning and ending that spas only one frame.