My phone has been hacked

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware if it is updated to the latest iOS/iPadOS unless you have intentionally downloaded spurious software or unauthorized apps directly from the internet and installed on your device or/and have Jail Broken. 

It (Hacking) also depends on how careful are you in sharing sensitive and valuable information pertaining to your iPhone such as Passcode, Password etc with your friends and family members.

Be careful when sharing the device's sensitive and valuable information with friends and family members.

Thumb Rules:

1. Enable  Guided Access before handing over an iPhone to kids
2. Don't share Apple IDs
3. Don't Jail Break
4. Don't share sensitive information pertaining to your device and payment methods
5. Beware of Phishing

Not a virus.

Not a hack.

Instead it is calendar spam that you inadvertently placed on your calendar by accepting an offer from a web site.

If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

You are quite welcome.

Stay alert, as those criminals are always attempting new ways to scam us.