Missing Beatmaps:
The Star Filter Method:
Quite often, the 'Filter By Star Rating' setting is the culprit for missing Beatmaps.
Make sure that these sliders in settings are set as shown above.
The Beatmap Sort Method:
Sometimes having the Beatmap Filter set to 'Collections' can cause no maps to show. Make sure to select 'No Grouping' to ensure that all maps are visible
The Reload Beatmaps Method:
Press F5 to reload all maps, sometimes it just breaks like that.