This issue is commonly associated with iOS devices following a major update to the operating system.
As part of the installation process of a new version of iOS, some users choose to perform a clean install, clearing your device's cache; unfortunately this also causes all content downloaded through Deezer to be lost.
In order to download your tracks again, navigate the desired album/playlist or track and toggle the Download button so the colour turns green, indicating it's enabled.
Note: downloading tracks can be performed via your mobile provider's network or wifi. Please check the Deezer app settings if you are concerned about your data usage and wish to disable downloading via your mobile provider.