A few things could have gone wrong from the viewer's setting becoming corrupt in some way to permission problems. With the lack of detail, I shall take a bit of a guess.
To check to make sure the service is actually configured to start-up properly; go to your services to ensure that service
eventlog (Windows Event Log) is set to
Automatic start-up type and that it is configured with the proper account. You might also need to tweak the recovery options.
To make sure the permissions are set properly for the service, go to
%windir%\system32\winevt\Logs and ensure the proper permissions are set for the account that is running the
eventlog service so that the process is actually able to use the directory.