Wow! This is a common question here in the forums. Its happened because under Admin->General you turned off the checkbox that says show all modules to all users when some of the modules dont have a group assigned to them.
so, goto: http://yourdomain/tiki-admin.php?page=general and turn it back on
now goto the admin modules screen and give each module either anonymous, or registered groups, or if you have created your own assign your own
then turn the check back off.
OH, to login point to http://yourdomain/tiki-login_scr.php
If you get stuck post back. or have a search for the other answers to the same problem