Compile Views in Oracle
If you ALTER a table that is referenced by a view, or even DROP a table, the view is marked as invalid and can't be accessed even if the changes to the table(s) would not make the view's code incorrect. 

In that case the view needs to be re-compiled:

ALTER VIEW some_view COMPILE;


More details in the manual:
https://docs.oracle.com/database/121/SQLRF/statements_4004.htm#SQLRF01104