I assume you are only using Google Calendar for your sync? (I use sync between my Outlook and Google Calendar.)
I would first try reviewing my settings on both the website and the phone. Make sure that the settings show sync: Go to menu>settings>accounts and sync>Google and make sure the sync box is still checked.
(I just noticed that my Google account states, "Sync is currently experiencing problems. It will be back shortly." This may be the issue?)