If you're in a tutorial with me, what you will say at this point is that you like spare, minimalist writing, and I will say, so do I, but the publishers simply doesn't buy novellas from unknown authors. If you want to get published, your novel needs to grow...
1. Missing scenes. When I read short manuscripts there are usually scenes that are referred to, but not written. Sometimes they're ones that seem vital to me as a reader. You'd only need 5 new scenes at 3,000 words each to hit 70,000 words.
2. Linear writing. If your novel goes straight to the point without much diversion it's going to be short. Try adding some diversions aka subplots.
3. Minor characters. Look for minor characters that could be developed more fully and given stories of their own. In Kissing Mr Wrong, one of the differences between draft 1 and draft 2 was that Briony (a minor but vital character) got her own story which developed throughout the novel in parallel to the main one.
4. Sparse writing. It's good for readers to have to do some work, that's how you engage a reader, but if they have to work too hard they'll give up and go read something else. Readers aren't stupid or determinedly dim, but reading is a form of entertainment, not hard labour. Sometimes you can over do the sparseness as you prune and prune. It's about getting the balance right.
5. Description. I'm not suggesting you suddenly shove great wodges of descriptive writing into your novel to get the word count up, but readers need a bit of description to help them imagine places and people. Some manuscripts look more like film scripts as there is no description in the dialogue sections.
6. Internal thoughts. There is a tendency for some writers to write in cinematic third person, ie no internal thought. In my experience this is usually a sign of a writer who watches film and television, but tends not to read that much. The novel reads like a prose version of a film script. This is a shame, as one of the fabulous qualities of prose is that it's the only art form where we know what another person is thinking. Add thoughts, add attitude. Your readers will thank you for it.
7. This is a desperate trick, and one that I tend to use in the horrible first stages of the first draft when I am obsessed with word counts, but play around with names. Mary Smith can become Mary Jo Fortescue Smith easily and will add to the word count without too much stress.
I am an under-writer, in that my first drafts are usually around 60,000 words. Each time I revise I feel I'm cutting like mad, but I also add all of the above (except No 7, that's a first draft trick) and my novels end up at around 95,000 words.