Overwatering. Take them out of the gravel tray immediately. Almost all plants can recover from quite severe underwatering, but can NEVER recover from only moderate.overwatering. Allow them to dry out for a few days, then water the compost but allow to drain freely. Gravel trays are best used to maintain humidity in hot, dry conditions. If pot bases are sitting in the water, the plant roots cannot get air. Many plants can survive many more degrees of cold if kept dry. Cold and wet together are a guaranteed killer. Almost all plants do better with less water in winter. My fuchsias only started to get a dribble of water on the odd warm day in Feb. They have to ask me for water at this time of yr. As do all of my greenhouse plants except seedlings. Never water all plants together. The same plant in the same pot of the same size can need different amounts of water at different times. ALWAYS feel the soil, look at the soil and the plant. Never water anything until the soil is at least a little dry on top. The plant needs to use the water before you give it more. How would you feel if someone forced you to drink water before you were thirsty again? The Japanese Tortured and drowned people that way in POW camps. It is the same for plants. Most prefer some degree of neglect to over-attentive watering, feeding and potting on.
If they mean enought to you to be bothered, you may be able to save them by re-potting into a mixture of potting compost, perlite and vermiculite at a rough ratio of 2:2:1. do not water for a few days after re-potting, but spray the leaves with tepid water if they look droopy. Once the plant has settled in it's pot, water lightly, allowing to drain straight through.The perlite and vermiculite will hold enough water from that for the plant to access until the next watering. In this way, you can save the plants, unless root rot has already set in, in which case, remove all dead roots, into healthy white tissue, then pot into the mixture shown above, and keep dryer for longer. PM if any further questions. I have saved many plants this way.
In general, the cooler the temp, or the less roots in a pot or container, the less water needed. The higher the temp, and/or the more roots, the more water. A very good rule is to wait unti lthe surface of the compost in the pot is dry before watering. In large pots indoors, you should be able to insert your finger to the first digit before feeling moisture before you water again. Plant roots need just as much air as water, and water drives out air. You could not survive under water very long. Neither can your plants. And sitting them in a tray so that water is always at their roots means that the water rises by capillary action to the top of the soil as it is used. At this time of yr, the plants are not respiring, not using much water, so gravel trays increase humidity, and possibly drown roots at the time when they need to cope with the odd cold spell by being dry. The most damage is done by water molecules within the plant swelling with extreme cold or frost. Young, soft plant tissues 'burst' under the pressure, and the plant will die. Hardy plants cope with this by taking up only the water that is necessary for suvival at the time of yr. Half hardy plants come from warmer climes and need us to balance the cold and wet for them. If they take a full draught, then get cold, thier cells literally burst and the plant dies. Keep them dry and they can cope at much lower temps.
In short, stop bloody watering! Also, plants can be gradually trained to need less water but still do well, other than some veg and fruits. If you always keep your plants slightly short of water, you save yourself lots of hassle in droughts! Pm me for more info, I learnt lots in my last job that has helped immeasurably now with outdoor plants..