can you please check the sqlquery ,whether you are able to get the data based on above query in sqlserver
If query is correct can you change the query as below and run and see what result you are getting
SqlCeCommand cmd = new SqlCeCommand("select SUM(GlassesPrice) as income from EyeGlasses where CheckinDate='" + date +"'", conn);
If you have any issue ...please ask