All my Calendars and events gone!

I literally just charged my phone today and when I went to go add an event into my calendar, all my saved events, birthdays and everything were gone!

My calendar originally had the On your IPhone option to add events in like the notes app but suddenly it’s missing and there seems to be no way to get it back

Please I’m so desperate, I had a lot of important dates, times and event in there that i genuinely don’t remember because that’s why I use my calendar but now I don’t know how to get it back.

My phone hasn’t updated or anything, it’s just all gone