Most of my contacts have disappeared from my iPhone.

Unlike some Android devices, contacts are neither stored locally on an iPhone nor in SIM. They are synced to an email account that you had Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other

If Contacts were on iCloud, try this --> Restore contacts, calendars, bookmarks, and more using ...

Else Add the email account that stores your contacts in the IOS mail app and enable Contacts.If you have a corporate email account added through a profile on your device, please check with your organisation's system administrator.

Were you able to check if your Contacts is enabled in the iCloud settings in your iPhone?

Settings > tap yout name > iCloud and see if Contacts is turned on there. If it is, was it only turned on after you accidentally deleted your contacts?

Some of our contacts are synced/saved with 3rd party emails. Do you recall removing an email account recently?

You can go to Settings > Contacts > Account and try to add back the emails you've deleted and see of your Contacts would show up

Checking your Contacts at iCloud.com is a good way to confirm if you have indeed synced your Contacts in iCloud or not. If they're not showing there, chances are, yout Contacts were not synced via iCloud at all. Or perhaps your contacts are associated with another Apple ID? Please note that your iCloud data is account specific so you have to make sure that you've signed on the correct Apple ID on your iPhone and iCloud.com