It will take several hours (up to 24) for all of your mail to show up after we move your account. If today is your migration day, it may just be that it's taking time for everything to show up. If it's been more than 24 hours, contact the Help Desk (via online support form  or 707.826.4357).
If you are missing mail that is sorted into folders, it may just be the way that Google displays folders (labels). Learn about Labels and how to enable the Nested Labels lab  so things will seem a little more orderly.