1. Create a new Constituent query.
  2. Leave the Criteria tab blank.
  3. On the Output tab, select Constituent Information -> Constituent ID.
  4. On the Sort tab, select Constituent Information -> Constituent ID (desc.)
  5. Run the query.
This will show you the last Constituent ID that is in use in the system. Open the constituent record(s) in question and manually add the next available Constituent ID.