I agree with sussamb that the two GPSMAP devices I have used for a decade have been brilliant, but they do have a steep learning curve.
Was the magenta line that you were following a track or a route?
If it was a route (i.e. not a track) that you had loaded, if it disappears again you could look at the Active Route app to see if the route was still being navigated? If not, you could restart the navigation.
If it was a track (ie not routable) that you had loaded, then as Slowcoach has said, a quick check in the Track Manager will determine if it is still set to "Show on Map".
Your Garmin can be set to Auto Pause the track recording when stationary, but this shouldn't affect the display of the route or track on the map that you are following.
To sussamb's point, I think that while using any routing activity other than 'Direct', the Oregon is limited to 50 waypoints per route, so this is certainly worth checking.