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Can you clarify.....
Have you deleted the xoops_session table or have you deleted the entire XOOPS database? Xoops_session is just a table, not a database.
Did you make a backup before you carried out the flush? If not, then naughty naughty you are!! I don't know know to help if you haven't, other then re-installing XOOPS on a test site, making a note of the tables values from that, then copying it to your live site using phpMyAdmin.
If you do have a backup, then in the case of you deleting the entire database then try just restoring your backup copy. In the case of just deleting the table, open your backup, copy all the values of the xoops_session table (using pen and paper) and then re-create it using phpMyAdmin or whatever database tool you like.