IOTA have just upgraded their wallet to 2.5.0. I logged into my 2.4.0 wallet without realising this and my balance was 0 with no history.
Solution
Download your updated wallet - https://github.com/iotaledger/wallet/releases/tag/v2.5.0
Read this guide - https://www.reddit.com/r/Iota/comments/7203ef/how_to_retrieve_iotas_with_wallet_250/