I assume that you've checked your phone's built-in calendar app (in case it's just Smooth Calendar playing up). If it is then just removing and re-adding the widget might be enough (the Smooth Calendar I looked up seems to just be a widget?).
Have you checked the calendar app's settings to make sure that your calendar is set to be visible? There will be one tick box in there that if unticked will mean the calendar disappears; if that's the problem, ticking it again will fix everything.
Otherwise, is your calendar linked to an account (e.g. a Google or Samsung calendar account) or is it device-only? If it's linked to an account the data should still be there in the cloud, you just need to make the phone resync them. Maybe check with a to see if they are still there.
If the calendar was only stored on the device, and isn't just being hidden by the app, and doesn't reappar after rebooting the phone (the "turn it off and turn it on again" solution, but always worth a try), then there may not be anything else you can do. But try the first few things here in case, as it may not come to that.