It has nothing to do with “steal”; I’m afraid many Verizon salesman are absolutely clueless. One of my associates was a support manager for Verizon stores. Several years ago Verizon fired all of their field support people, and replaced them with minimally trained sales people. My associate was laid off after 20 years of service with the company.
The iPhone, as with most computers also, are not the primary repository of contacts. The email servers for your email accounts are the master repository of contacts; your iPhone syncs to the email servers to display contacts. If you delete an email account or lose access to that account that is hosting some or all of your contacts those contacts will disappear. So most likely the contacts that you lost were hosted on your Comcast account. They should come back if you reconnect to the email server. As the password has changed, the easiest way to do that is to go to Settings/Mail/Accounts, tap on the Comcast account and delete it. Then restart your phone, go back to the same location and add the account back.
Next, go to Settings/Contacts and change the Default Account to one that you will never delete.
Optionally, get an 3rd party app like Groups and move all of your Comcast contacts to the default account. If you have a Mac you can also do this transfer on your Mac.