Bugs! Something got in there and is sapping vital plant juices. Don't bring it into the house & it should be quarentined until the issue is resolved. I can't say whether it's in the leaves, stem or roots but the timing is very suspicious. Apply a systemic insecticide following the directions meticulously. Fresh new potting mix wouldn't hurt either.

welcome to GoY now for a few questions to help us come to some idea of what has happened.

how old is it? I find that they are not that long lived.

Is it really close to a wall? if it is it could be due to lack of water especially if you have had frosts lately.

Weirdly if your garden is water logged that will also cause wilting as the fine roots die and no water is taken up. So if waterlogged try to increase the drainage.

Alternatively have you been digging close to the base of the tree, this would cause damage to the fine roots that actually take up the water.

it is unlikely to be insect damage this early in the year if you are in the UK.

Did it have any toadstools at its base in the autumn?