Everything I’ve ever read about submitting your novel for consideration to agents or publishers says this:
Keep writing during the querying stage. Work on a new project. Keep the creative juices flowing.

But, why? Querying is tiring work! Researching those who might be interested, searching for that key word on a profile that says, “Yes! That! They like weird, gothic, atmospheric, Southern, twisty-troped stories about ice cream! That must mean they’ll love my book, right?” Crafting your letter to match the tone of the agent bio you just read. Carefully going down the checklist of requirements … oh, this one wants exactly 27 pages, with only a one-page synopsis but also a description of the last plate of sushi I ate … (I’m only being slightly facetious, here.)
Well, dear fellow writers, it’s because querying is so exhausting that you must work on something else. Not with a strategy in mind, not with a battle plan that goes like, “If my first novel won’t sell, this second one will.” Although it does sometimes happen in that way, that’s not your purpose in dancing your fingers on the keyboard on the daily.
No, you need to keep writing for yourself. Because it’s how you pay the utility bill for your soul, friend. Keep writing to tell yourself the story you want to hear. Ultimately, if you’re writing for another reason, you’re doing it wrong.
The rewriting, the editing, the polishing and crafting … that part is for your story to see the light of day, to potentially make a living for you. But, you can’t get there until you get the messy bits of your story out, first, for only your enjoyment.
Want to read about this concept from somebody who really knows what she’s talking about? Read Big Magic by Elizabeth Gilbert. Incidentally, it’s a book that can help pull you out of a querying-induced creative slump, too.