The autocomplete cache is created from all the recipients in the e-mails you have sent out. Occasionally this cache becomes corrupted so that autocomplete no longer works.
In a best case scenario you make backups of the .nk2 file so you can simply restore the backup.
.nk2 is the extension of the file that holds the addresses of your Outlook autocomplete cache.
Download and Install
RebuildOutlookCache This is a zip file save it and extract the installation files. Once it is installed run the program.
Next click on the "Auto Rebuild" button, this will process your "Send Items" and your "Contacts" Folder
If you like to scan other folders then select those with the "Select Additional Folder" button before you press the auto Rebuild Button.
Outlook may warn you that a program is trying to access e-mail addresses. Set it to allow access for 10 minutes and click OK.
After a while it will finish and you can enjoy the autocomplete again.
Dont forget to close Outlook before you reboot. It seems some versions of outlook dont save the .nk2 file if they are killed by a reboot.