The suggestion made by rpf is a good one and will prove whether or not the remote control "sender" is operating or not.
If the remote control sender is working, you might check to see if something is obscuring the window on the fireplace that "looks" at the remote sender and receives the signals. This window may be round or square and covered with a bit of plastic that looks dark or deep red to the human eye. TV sets have them on the front of the cabinet. This is a filter that only lets through the infrared light and so lessens the chance of the electronics being "confused" by flashes of visible light. If something is covering this window, the infrared signals can't get through and the remote won't work.
Is there only one remote sender, or two, one for each fireplace? If there are two it's unusual for both to go bad at the same time.
Are they the remote senders that came with the fireplaces or generic replacement ones? If they are generic replacement ones that are programmable, they may have lost their programming over the summer when their batteries went flat.