Reinstall flash, pretty confident to say that's what's blocking it from playing.
Adobe - Adobe Flash Player
Other reasons than that may be that some third party codec or modified player of the sort is taking over your browser. Or the PC is too old (hopefully not). Or you're using the 64 bit version of a browser without 64bit flash installed for it.
Hulu is probably the only place I would check for replays of shows online.. but I usually don't do that, I just get a copy of the complete show by other means if there's something I wanted to see.