How do I recover lost calendar items, birthdays, and reminders after a iOS 26.5.2 update?

Keritommo wrote:

Since 17/07/2026 when phone updated I’ve lost years worth of calendar items as my calendar is linked to my btinternet account. My 82 year old dad missed a very important eye appointment at the hospital today as a result of me not getting a reminder! I’ve lost years of information birthdays and special birthdays and lots of medical appointments for me too.. can anyone let please as so far Apple can’t and neither can BT….

[Re-Titled by Moderator]

Original Title: Lost all calendar items - 14 pro max appointments past and future, birthdays past and future and reminders I put in calendar. 26.5.2 update!!

IF it is linked to btinternet account, then you need to re-able calendar.

If that doesn't work, you'll need to escalate the issue with BT support, since you used their account to sync to.