When I deleted my Yahoo email account from my iPhone, I lost most of my contacts

That’s because those contacts were being synced with your Yahoo account.

Re add the account to your phone.

You were using Yahoo as your default Contacts source, so when you deleted the Yahoo account the contacts went with it. As your Yahoo account is the only source for those contacts you will have to add it back (if you can), or access it some other way and copy the contacts to a different contact source (such as iCloud).