If you have already downloaded maps but the screen stays white, there are three usual causes.

The map is not centred on your position. If you downloaded the map for where you are, make sure the map is following you — tap the round follow button in the top-left corner until the icon is a filled arrow or compass.

The SD card is not being read. If your maps live on an SD card and the card is not detected, the map goes blank once you zoom in past the world overview, and DMD2 tells you the card was not detected. Your maps are still on the card and nothing has been lost. Power the device down, re-seat the card and start up again. Do not re-download in this state — you would fill internal storage with a second copy of maps you already own.

A map file is corrupted. Either the download was interrupted too many times, the storage changed, or the file is simply incomplete. Download that map again. If you have several maps, switch them all off and enable them one at a time to find the one at fault, then re-download just that one.

If the map is not blank but broken up by white lines, see the separate answer on white stripes.