Check the "Email to receive critical site notifications." sometimes there is an explicit message.

Depending on how often you have your cron job to run, this may not happen immediately. You can always check to see when it ran last by going into Studio > Dashboard > and click the down arrow next to Host Tools and click Server Audit. 

Scroll down to the Site Settings area and see when you cron job ran last versus the time you click the upgrade button. You should get an success/failure email once it's done.