There have been a couple of sites that didn't get enough candidates. The first thing SE did was to extend nominations for another week. I assume this was accompanied by current mods and others trying to generate more interest among prospective candidates.
One site had an election cancelled after this failed, but I don't know what happened with remaining mods -- whether mods who wanted to step down stayed longer or the site ran with fewer mods or what.
Because yours is a pro-tempore election, not a graduation election, SE can still appoint mods if they want to. That's how pro-tem mods have historically gotten the job; pro-tem elections are a new thing. However, if your election is extended and still fails, I strongly suspect that y'all will be having a conversation with community managers about paths forward before any decisions are made about new mods. If your users are too shy to put themselves forward, that's a different problem from if they're just not interested. Y'all will need to figure out what's going on there.
One of my sites (Workplace) is currently having an election, so far with one candidate. We were concerned about candidate turnout because our last election was kind of rocky, so mods put together some meta posts about what the job entails, trying to be clear that it's not scary and there's a support team (the other mods). It's too soon to say whether it helped and I don't know how active your meta is, but if you have a community there, you might try "selling" the job. (Update: four candidates nominated.)