I've got a novel(la) of 33,000+ (130 or so manuscript pages) words that I would like to build up to 60,000 or so words - but I'm having trouble restarting the project...
So my question is this: what should I be doing to begin again?
I don't have a problem with expanding a novella or short story into a novel. It can definitely work. For example, Orson Scott Card's "Ender's Game" was originally a short story published in Analog Science Fiction and Fact magazine, and he expanded that into an incredible novel, plus followed it up with additional books. There are no doubt other examples to choose from. However, just because you can, doesn't mean you should, and I imagine that not all leaps from short story/novella to novel are as successful as Ender's Game.
I would suggest that you start with a simple question that may be difficult to answer: What is there that you are left wondering about at the end of the story? Perhaps you're left wondering about events before what's depicted. It could be that the story starts in the wrong place, and should have started earlier in order to cover several events that define your characters and heighten the conflict more, or explain/expand the plot arcs better. Perhaps you've reached the end, and you're left wondering more and more about a particular character: how they got there, who are they, what's their history. You may discover you're telling the wrong story, and you should be focusing on a different character because you're asking more questions about them than anyone else. Perhaps you're wondering more and more about the setting, or the relationships between the various characters ... it could be anything, but the main point is that you think there's something fundamentally missing that needs to be told.
In the case of the Ender's Game short story, it essentially covers what's the core basis for the ending of the first book (although obviously it's different since in writing the novel, OSC changed a lot). How much more meaningful is that ending once we've lived and breathed Ender's life until that point! The relationship with his brother and sister, the friends and foes he meets, the terrible inner struggle he faces. Clearly, even though the short story stood on its own, the character of Ender deserved to be explored, and with it, all the events, relationships, trials and tribulations that lead him up until that final point.
Not every novella or short story should be expanded, and can stand on their own. My advice is to read the story afresh, or give it to a few others to reread, and see what questions are in the back of your mind. It could be questions about the setting, the characters, the plot. Try identify what's missing, or left you wondering, and focus on that. If you're not wondering what's missing, or asking questions about characters and their past, then it would suggest that there's nothing more that needs to be added, in which case, consider editing and whittling your work instead.
The question is, why do you want to add the extra words? If it's a complete story, then IMHO the only reason you should be adding to it is because you feel that it's still lacking in certain elements, not because you want to reach a certain word count.
Perhaps it's a character who you feel needs more fleshing out because they play an important role later. Pick one, then see what elements of their personality you'd like to explore further. For example, if you have a main love interest who feels flat and generic, think about other interests or activities he/she can be involved in that will affect his/her relationship with the protagonist.
Linked to that, maybe there's a particular story thread that feels rushed and could be strung out to increase the tension. Consider who else can be thrown into the mix to complicate things, or maybe additional scenes that provide exposition to the key scenes and get the reader more emotionally involved.
If you're halfway through and the story is unfinished, consider what other conflicts you can throw at the characters. Don't go throwing everything but the kitchen sink at them, but consider what conflicts will make your ending that much sweeter when the reader finally gets there. I think this has been said many times, but if you're stalled for ideas, add more problems and see what the fallout is from that. Or throw in a new, out-there character and see how the others react. You may not use the additional conflicts or characters, but they're good for getting ideas going again and reconnecting with your world.