Sorry to hear about your issue :(
Unfortunately, we cannot get the contents unless you have a backup. Let's check some basic settings first.
You mean to say you lost all the calendar entries or only a few?
Let's check this first. On your iPhone, go to Calendar app.
Now you can see three tabs at the bottom - Today, Calendars and Inbox.
Tap on "calendars" tab there.
Now, is everything having a check mark next to it? If not, click on the option "show all calendars" and then click on done.
Let's see if that resolves your issue.