Completely clear your internet browser's cache and cookies.
If you are unsure of how to do this, please review the instructions here:
This issue appears to be most prevalent on Mac OS X but is known to also occur on Windows.
Please also try:
Temporarily disabling any firewall software you may be running - as this can intercept the necessary connections between Sketchup and the licensing server.
It this fails to resolve your issue, please contact Sketchup Pro support directly, who will be able to provide support relevant to your particular system and setup: