I know that quite a few of us use the Topo GPS app as a navigation aid. Mine crapped out on me the other day (iPhone7). It happened after the latest update (Version 5.1). It took a while to work out what was going on, but it transpired that I needed to re-download some of the offline tiles. In short, if you use it, recheck the appropriate tiles are downloaded, preferably at home or somewhere else with a WiFi connection.
