It's not very common for a hen to quit. Most of the time she's just taking a long bio break and will return. Put her back on nest and see if she settles back down. If she does leave for good, there are a few things you could to extend the warmth.
Put eggs in an insulated container - like lunchbox or mini cooler - with a hand warmer packet near eggs..... or heating pad...... or microwaved rice (Place dry rice in sock & close with rubber band) BTW- I like this for headaches.
Remember the goal is to keep them at 99.5'F, so pretty close to human body temp. Don't make it too hot. These things will help keep eggs warm while you go to store.
Eggs can also go dormant, so if they do chill to room temp, there's a strong chance they'll hatch a day or two late. I once had a hen leave the nest and return to the wrong one (when we had snow on the ground!) It was over an hour & the eggs were pretty cold. I put her back on her eggs and all 3 hatched.
A new & easy incubator is the Nurture Rite 360. It's so easy to set up, use, and clean. It also has good view for hatching.
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If you really don't see that you'll have much future need for an incubator, then ask around to see if you could borrow one. Many schools have them and only use them once a year. Any BYC member near you? Other friends/family with chickens? Home school families? Free is always good.