There might be an app for that. I would start there. U can call ur service to look at ur phone records. Or u could ask the NSA to help u out

Lost how? Was it in your contacts? What kind of phone? Ever called that number? Ever called you? Did you sync your contacts with an external program, like gmail?

You could get a record of your phone call history, it won’t list the contact name but if you really want it you could go through and cross them off one by one.