Keep tapping the 'Esc' key during boot to bring up a menu with the
kernels installed then select an earlier one to boot into. If you only
have the one kernel then choose the Recovery Mode option to boot into
then close down and try a full boot again.

Earlier kernels are similar to Windows System Restore and Recovery
Mode is similar to Windows Safe Mode.

BTW - it's not a good idea to try an upgrade a non-working OS of any
kind... that can be a short trip to 'totally hosed and only a clean
install is going to fix it' !!

When upgrading the data in the Home partition is usually transferred
but might not be. To be safe it's always a good idea to back up
everything first, but we all back up regularly anyway don't we, and
it's useful if the Home directory is on it's own partition ( that
means a manual install and can be daunting the first time you do it,
like most things ).