Settings > Accounts & Sync > Click on your google account > Make sure what you want checked is checked, and they will sync
If they are all checked, and still won't sync, make sure you're phone is connected to some form of internet. 3G, 4G, Wifi, w/e, then retry the stuff above.
If that doesn't work still, are you rooted?
-If so, reflash your current ROM or flash a different ROM. This could be a bug?
-If not, you can try to remove any new apps you've installed right before this issue came up. If that doesn't work, then you can try a Factory Reset.
When it comes to software problems like this, it's really hard to troubleshoot. :-\