This is fairly uncommon, but should it happen, take your rat to the vet. He/She will advise what to do, if anything can be done. If the tooth is lost, then a misalignment can occur. This can result in loss of appetite and weight loss.
If infection occurs around the root, then seek your vet’s advice promptly. This can be noted by swelling around the cheek area, but this is not the only cause of that type of swelling.