My upper single tooth is cracked horizontally just over the gum line. Is it possible to repair it?

The gum should be probed around the tooth to determine the level of tooth fracture and bone level. In some situations, crown lengthening can be done to make the tooth restorable. It all depends on exam findings. In some other situations, extracting the tooth and placing an implant offers a better more long term solution. 

Hi ruk ruk khan,Unfortunately it's not possible to tell without more information. I would recommend you see a dentist to have it evaluated to get a better answer. Some cracks render the teeth non-restorable, but many cracks have treatment options to stabilize the tooth, so you should see a dentist to be evaluated.Good luck!