Changed host without DB backup, now old wordpress lost DB server

Old host having 2 domains, one they have given as default and another i bought at 1&1.
using 2nd domain only for running WordPress & its database. I wanted to move 2nd domain and wordpress to other host provider
I have just changed 2nd domain’s name-servers from old host to new host, after successful name-server change, old host’s phpmyadmin throwing error like “#2002 unable to login” i dint taken backup of old database.
and primary-url/wp-admin showing error like “Error establishing a database connection”, What i suppose to do now?. Changing NS back to old host will work?
addition: tried to change WP url from 1st domain to 2nd domain in wp-config
tried database repair. nothing worked.