If you have been charged for the download you will be able to do a limited number of re-downloads from MixRadio.
On your Nokia Lumia: Open the MixRadio application > find the track in Find artist, music or gigs and click the Buy link - it will be downloaded with your existing redownload rights instead of charging you again.
Please note that the same track or album can appear more than once in different versions in the store, so make sure you retry the same track or album.
You can also access your download history at http://www.mixrad.io on your PC, Mac or tablet. Go to http://www.mixrad.io and sign in. Select Settings > Download history. MixRadio does not guarantee a recovery service for music downloaded from the service.