Hi and welcome to beyond blue.
firstly let me echo the comments made by Sophie_M - I can tell from your post the relationship between your father and yourself was very close and special, something to be cherished.
You posed the question Why now? Some questions are difficult to answer. The grieving process may occur after the event, or the much later. There is not set timetable either. For today, it could have been a sound, a smell, or something that triggered my memory. Whatever it was, for you is also normal. Sophie_M provided information on some groups to get in touch with - talking to your friends perhaps asking them for support and perhaps a local support group. There are ways to get through this period of feeling low. Other people here will also lend their support to you.
Listening to you tell your story,