I had to reset my phone; how can I repair it to my Apple Watch?

If you had to take your iPhone through the initial setup process again it should have asked about your Apple ID and offered to restore your last (iCloud) backup. This should have seen the Watch configuration and offered to restore the connection to your Apple Watch. At least, that was what happened for me when I had to do an unplanned replacement after my iPhone jumped from a pocket and went swimming.

Apple Watch normally backs up to your iPhone, and from there in your iCloud backups.

If you had an iPhone backup but didn't restore it, I would reset the setup process and include the backup. If you didn't have an iPhone backup all you can do is erase the watch and hope before you pair and setup again - but I wouldn't be too hopeful about recovering settings because I suspect nothing would have been saved.