Many of my contact have disappeared from iPhone

Contacts that are displayed in the Contacts app on the iPhone can be stored in several locations including on the device itself, or more commonly from one or more servers such as iCloud, Google, Microsoft, etc. Sometimes, when contacts disappear, it simply means that the services that they are stored in are either not yet added to the phone, are not yet logged in, or the account is deselected in the Contacts app.

Have a look at:

1. Settings > Apps (at the bottom of settings) > Contacts
2. Touch "Contacts Accounts >"
3. Make sure that the server/account where you had your Contact stored is listed
4. Touch that account
5. Make sure that "Contacts" is enabled for that account

Additional information is available in: Add or remove accounts in Contacts on iPhone - Apple Support