Hello zuirti, okay so it was moved to the basement. So it maybe a combination of things. Depending how late it was moved during winter from outdoors to indoors, it could have been exposed to cold. Then if light is not sufficient at the basement, plant suffers even more. Too soggy media when it does not dry out quick enough, root rot.
Basement growing is done by a lot of gardeners during winter, but they augment it with artificial light, and got to be mindful of watering, it takes longer for water to dry out indoors, so easy to get rotted roots. Rotted roots- plant cannot take in moisture, no light-then photosynthesis is hampered, plant suffers a lot.