A frequent question is how do I recover my database password?, see here
The typical process is always the same
If you haven't just go to the next point, otherwise you will need to start mysql with skip-grant-tables More info here
You can access using the debian-sys-maint user on most of debian systems, you'll e able to recover your user from there.
mysql -udebian-sys-maint -p`grep password /etc/mysql/debian.cnf|head -n1|awk '{print $3}'`
To recover that one you have to edit two files imscp.old.conf and imscp.conf and delete the values on both files for the entries:
DATABASE_HOST = DATABASE_PORT = DATABASE_NAME = DATABASE_PASSWORD =
After that re-run the autoinstaller that will ask you the user and password to use to manage i-mscp:
perl ./imscp-autoinstall