The simple fact of the matter, as several have said above, is that some stories can only be so long. If you go in and consciously attempt to pad the story to get it up to some artificial word count you're only going to hurt the story and probably your chances of it ever seeing publication.
However, and it's a big however, you can go through the story line by line looking for plotholes and scenes and situations which need further expansion. Let me give you an example - in my first real attempt to write a book I set myself a conscious goal of 50 thousand words. I literally blazed through that book hitting 50 thousand words on or about Day 26 or 27, but it wasn't done. I forged ahead and by Day 35 had finished the book and hit 65 thousand words.
Now, that still wasn't large enough to make a novel in today's publishing world, but who cares. I finished it! Every idea got into the book. All my research got into the book (too much of it really). I set that book aside and had the satisfaction of knowing I'd finished it.
But I also knew the book wasn't done yet. Six weeks or so later I opened the manuscript back up and started going through it line-by-line. I found major plotholes and areas of info-dumping that were just awful. I started the rewrite. Four and a half drafts later and the book is up to 80 thousand words and still growing because there were entire scenes left out that created humongous continuity issues. The book looks like it will finish a 5th draft at around 85-90k words. After that there will be a 6th draft and that will likely mean cutting the whole thing back down to around 80k.
80k is still kinda' small in today's publishing markets where authors publish massive tomes full of drivel, silly character descriptions, and tons of info-dumping.
Here's the truth of the matter - skinny and tight is better than bloated and wordy. You've got a nice tight little novella. There are markets for novella's. You finished it. Pat yourself on the back, set that puppy aside for a few weeks, and get to work on your next piece.
In a few weeks go back to that novella and see where you can easily pad it out to fix continuity issues, where you left out important scenes, where character description and dialogue can be fixed, etc., etc., etc. Maybe you'll have to cut it. Maybe you can expand it. Maybe it's perfect just the way it is. Be patient and fix the problems as they come your way in your writing. Chances are extremely high it's going to take you 2-3 years to sell it if you sell it at all.
Impatience and frustration and worrying about some supposed word count limitation are big time killers of a writer's inspiration. Take what comes your way and run with it and don't worry about some artificial word count. Stories have a way of making themselves the lengths they're meant to be.