The warm spells of weather (above 12°C) are causing problems for bees, the bees increased activity means that they are using up precious stores quicker than anticipated, subsequently, colonies are starving!
Check NOW, how heavy is your hive? Remove the roof and pick up one side of the hive, is it surprisingly heavy or surprisingly light to lift? if the hive is surprisingly heavy then your bees are probably OK, if the hive feels light then you need to act NOW. Peep into the feed hole, are the bees moving? If you can't see anything then crack the crown board and look in between the frames, are the bees moving?
Bees eat honey which is fructose and glucose, whereas sugar is mainly sucrose, and in cold weather bees will have to work to convert sugar syrup to fructose, so feed them fructose in the form of an Ambrosia™ a feed which bees can benefit from immediately, without effort. When bees are cold they cluster so any food will have to next to the cluster, feeding candy between the frames and the crownboard can save a colony while the temprature is below 10°C, when the weather warms up you can use a contact feeder, rapid feeders don't work efficiently much below 16°C.
Local Ambrosia Supplier ←Click.
Remove all the frames from the hive and then all the dead bees from the frames, making sure you dispose of the bodies, if there are lots of dead bees then catch them on newspaper and dispose of them by burying them or composting them. Scrape the propolis from the frames and hive, and scorch the inside of the empty hive with a blowtorch before reassembling. The good beekeeper then sterilises the dead hive with Acetic Acid. Don't worry, in a few months you will have plenty of opportunities to fill the hive with bees again!