I used to have lots of names pop up when I click on the "To..." button of a new email. Now they are gone. What happened?

Greetings, JayMulkey !

To autocomplete feature when you click on To field is compose of Address Book and cache files.  You will need to Enable Contacts Database As An Email Address Book in Outlook 2003

1. click Tools > Email Accounts
2. click on 'View or change existing directories or address books' option.
3. click 'Next'.
4. click on 'Add' button > 'Additional Address Books' option.
5. click 'Next'.
6. click on the 'Outlook Address Book' option.
7. click 'Next'.
8. click on 'Finish' button.

9. click on the 'Contacts' tab
10. right-click on the 'Contacts' option on the left-hand side (under 'My
Contacts').
11. click on the 'Properties' option - see multi-tabbed dialog.
12. click on the 'Outlook Address Book' tab.
13. ensure that the 'Show this folder as an e-mail address book' option is
selected.
14. click OK.

Your Contacts database should now be active as an e-mail address book.
http://domino.lancs.ac.uk/pub/usghelpdesk.nsf/9fb9e62b73055de48025680100698378/c8dbe9bb81766cf080256e5800438856?OpenDocument

If still no joy, the cache names are kept in a .NK2 file.  With Outlook close, search for *.NK2 and rename the file.  When Outlook opens, a new one should be created.

.NK2 file is located in
C:\Documents and Settings\<user>\Application Data\Microsoft\Outlook folder. It is a hidden file. Go to any folder and select Tools > Folders Options > View. Check "Show hidden files and folders".  Click OK.

Best wishes!