This is a great question, and here's my tuppence worth.
First of all, don't panic! It will come, it will.
A few years ago, I had a serious block, for months, I even wrote to gromet, saying, I was done, finished. But it came back, something triggered an idea, maybe a TV programme, or something in a book I was reading, somebody I met, something I saw in the street or shop, a person, an interaction, but it did come back.
For what it is worth (probably nothing) I get most of my ideas at night, in bed. It's pitch black, completely silent, there are no distractions, I don't sleep much, so this is a perfect time to ponder. By the time I have come up with a scene, or a character relationship it's already written in my head, and I can get it down at the keyboard later, I can do 1,000/1,500 words without a break. I can't sit down with no ideas and stare at a screen, not a chance, that takes me nowhere.
On another note, when I start a story, I have no idea where it will end. Not a clue. I would find having an ending in mind very restrictive, forcing me somewhere that I might not want to go. The story will take me where it wants, characters will come and go, and even change. I like writing longer stories as it allows me to develop them. But that's me, we're all different.
Back to topic, don't panic, you'll be fine, it will come, quite likely completely out of the blue. Forcing it will not work, so don't "slam it together," that will disappoint you, and the reader.
Good luck!
Rbbral