I'm having the same kind of problem, several actually, even though I did have maintenance mode turned on before migration.
1. First I got a blank page after login, which seems to have been fixed by increasing memory limit from 32M to 128M in php.ini
2. Now I'm getting this error in IE after login: Coding error detected, it must be fixed by a programmer: moodle_database::update_record_raw() id field must be specified.
3. And these errors in Firefox: Table "config_plugins" does not exist; and Table "context" does not exist.
4. And just now I got an error from cron saying !!! Table "modules" does not exist !!!
Don't know what's going on. I'm guessing it's something to do with the nameservers not updated fully yet. So, I'm waiting.
Plus, there seems to be something wrong with phpmyadmin on the server, so I'm waiting for the hosting providers to check that out.
Anyway, had maintenance mode on, but still getting plenty of errors.