Try downloading another camera app such as opencamera. If it doesn't work either, back up your information, try a factory reset. If the factory reset doesn't fix the issue, you likely have a hardware failure.
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But this is a Galaxy S8, it should still be under manufacturer's warranty. You should just backup your data and send it back to Samsung and let them figure out if its software or hardware.
Try downloading another camera app such as opencamera. If it doesn't work either, back up your information, try a factory reset. If the factory reset doesn't fix the issue, you likely have a hardware failure.