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.
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.
What shall I do then? Thousands of students are waiting!!!!
I found this in one of the docs on how to determine php/debug errors when you can't login. Add the following lines at the end of config.php, and then try to login again:
ini_set ('display_errors', 'on');
ini_set ('log_errors', 'on');
ini_set ('display_startup_errors', 'on');
ini_set ('error_reporting', E_ALL);
$CFG->debug = DEBUG_ALL;
See if the errors tell you anything more specific.