1. Are your OS maps still appearing on your unit?

2. if not I would take the micro SD card out of your unit and put it back in again.

2. If this does not solve it you need to make sure the map data is still on the card.

If you take the map card out of the unit, put it into the SD adaptor it will have come in, this will plug into an SD slot on a computer.

When you open BaseCamp you should then get an option to select the maps, if so, good news as the maps are still on the card.

If they are just put it back in your unit and see if reseating that card rectifies the problem.