It may be considered unwise to leave your PC on for long periods when unused. The PC is still `live` electrically and is using power, still `warm` and accessible. As a result, there is the risk of damage from electrical power `spikes`, perhaps fire could result and of course it is not particularly `green`.
Whether there is a real problem with it not resuming after long periods of inactivity or whether this is a `feature` of the OS I have no idea. But, it would seem reasonable for it to resume unless there is some form of software problem; bad graphics drivers have been known to cause this kind of anomaly in XP.