Something is wrong calendar alert

Last night when I was on my iPhone I got a calendar invite banner across the top of my screen saying SOS there is something wrong and when I opened my calendar this alert was now on every day all day. There’s no option to delete either. I checked my emails and found the attached pic email in my junk folder. My husband said to delete the calendar event but I couldn’t, I could only accept, maybe or decline. I pressed decline and it said I couldn’t delete the event. Has anyone else had this and how do I fix it? Does someone now have access to my phone?