Restarting the phone to get the camera to work sounds like something is keeping your camera running in the background. Have you installed any apps that require camera operation recently? Check your running app list and see if there is an app there that running that has camera permission. Once you solve that your camera issue should clear up.
What it sounds like is an app is running your camera in standby and it's probably ok for awhile, maybe a few minutes or even an hour. Over time of constantly being ready it's probably freezing and causing your camera to not respond to the nave camera app. If you were to open the app causing this issue it might activate or it might freeze as well, it would depend on the glitch cause. So when you restart the phone it essentially shuts down the app taking control giving your camera app back its control.
If you are having a difficult time finding the responsible app, try sorting your app drawer by recent and looking at the first 5 or so apps there. These will be the last ones to be installed on your device and will hopefully reveal the problem app. You may be able to reinstall the app and clear up the issue if it was a bad install.