One of our administrators mistakenly deleted three contacts while trying to remove them from a distribution list. I restored the three contacts from trash, but didn't notice that the three restored contacts now no longer showed a Drupal User ID next their Contact ID in the CiviCRM summary screen for the contact.
The users were unable to sign up for events (you have to be have a membership to access signup), and when we realized this, I found each User, clicked the Edit button, then I clicked the "LMGA Member Info" button in the upper right of the Drupal User Edit screen. Just opening that tab showed the CiviCRM info for this member, and then I found that the CiviCRM summary screen for this user now showed Contact ID/User ID. The link seemed to have been restored by Drupal accessing the CiviCRM tables.
I think all is well now, but I'm unsure of what actually happened and why. Can some kind soul please enlighten me?