Hi, welcome to GoY, if it's a hardy fucshia, it would have been better planted in the ground, by taking it into the greenhouse, you have made it "soft" which means that it should have been gradually acclimatised to outdoor conditions, and the sudden change in temperature has made it react, by dropping it's leaves, all you can do now is wait, and eventually it should start to produce new leaves, but it may be well into late spring or early summer before this happens, Derek.

It rather depends on whether the leaves that have fallen are the ones its kept all winter or new ones grown this year. If they are last years don't worry. The plant will probably send up new growth from the base and may even put out new leaves on the old stems. Keep it in a sheltered position and see what happens.

If you want to overwinter fuchsias indoors another year you can either keep them growing but acclimatise them to outdoors gradually a bit later in the season, or you can let them get pretty dry and keep them in as cool a room as you have. They will lose their leaves but grow again when you start to water in the spring.
As Derek says, hardy ones are normally OK outside in the ground (not in containers in case of hard frosts).Plant them with the crwons 3 0r 4 inches lower than the soilk level in the pot which again will protect the crowns from frosts.

Crwons and soilk ...time I went to bed! Forgot to say it would be good to cut back all the stems at least half way back. New growth comes on quickly and flowers rather more freely than on last year's wood.