Hi,
Did you have Permalinks enabled? If so, that might be the problem. Enter PhpMyAdmin (usually through you Web Panel -Plesk, cPanel...), your Osclass database, run this query:
UPDATE oc_t_preference SET s_value = '0' WHERE s_section = 'osclass' AND s_name = 'rewriteEnabled';
(If your tables have other prefix than the standard "oc_", change that in the query).
Then try to enter your Admin Dashboard to complete the upgrade. If successful, re-enable Permalinks there.
Regards