Mongo hangs in production application, no errors

It appears that my mongoDB is hanging during queries in my production application which serves about 40,000 users. My question is, can a local standalone mongo server handle activity of this many users, or would I need to create some sort of cache so the database isn’t constantly being queried. I’m not sure what else to troubleshoot or look for to resolve this, but basically my code just freezes in these queries, no errors, just stops running.