Most novels should run a mininium of 90K, and I've seen agents who say they'll reject something 30K-40K. That being said, I do have a tendancy to run short and end up adding. My last one was 15K too short, and I had to come up with ways to add it. My very first one was like yours. So this might help:
Think about how to restructure the storyline to bring in more developments, complications, and subplots that will both expand and enhance the story.
If you're one of those people who uses narrative to punctuate dialogue (there are people who write a lot of dialogue, then use the narrative to identify who said what--and don't use it for much else), then work on your narrative skills. Describe mood, setting, the character's emotions, whatever helps advance your story.
If you've left any unanswered questions because they were hard to do, go back and work through answering them. For example, in one romance novel, a woman really wanted a particular job at a particular company. The unanswered question the writer ignored was why that job. It was never explained. Something like that could really add a great deal to your story (and add pages).
Look for scenes where you can expand on them. More excitement, more tension, more something.
If your main character doesn't have a buddy, give her one. It's a lot easier to write scenes if your main character has someone to talk to.
