For Invision Board, there's a very important file 'config_gobal.php' that contains key data for the board. I cannot offer an immediate solution, but I suspect that this has something to do with it - either you've uploaded a newer version of this config file (and thus it doesn't contain your specific forum data) or you've somehow deleted your login. See if you can access the admin area of your board by going to - your url/forum/admin.php
The best suggestion I can offer though is go to the Invision Board support forum. There are hundreds of people there who offer helpful and immediate advice:
http://forums.invisionpower.com/