A fan for a DVR5100 must be purchased.
Pelco Video Management
DVR5100 hybrid-video recorder, all versions
Possible causes include (1) hardware failure, or (2) the need to obtain spare parts for future contingencies.
The part number for a replacement DVR5100 fan is FN01-2223-2400. There are two of these in a DVR5100. So, if both must be replaced, order two.