If the ULS log files remain empty (0 KB) take the following steps to resolve:
- Make sure you have enough space in your hardware driver.
- Check timer and tracing service accounts information, the tracing service must use the account as Local Service.
- Reset the timer service account password. Restarted the tracing service.
- Check Diagnostic logging under Central Administration site, you can refer to:http://technet.microsoft.com/en-us/library/ee748656.aspx
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- Check the permissions on /12/logs folder
And these blogs may help you:
http://sharepointlearningcurve.blogspot.com/2010/04/sharepoint-2010-uls-problems-logs-are.html 
http://www.eiben.weite-welt.com/2011/08/no-uls-logging-in-sharepoint-2010/
Taken from forum thread: http://social.technet.microsoft.com/Forums/en-US/sharepoint2010general/thread/e327f833-5e43-42d1-a330-f2d16c41106a
Check the following Wiki page for updates: http://social.technet.microsoft.com/wiki/contents/articles/9340.sharepoint-2010-solving-problems-with-uls-log-file-generation.aspx