In a ispconfig master slave setup, not all information from the master database is transferred to the slave(s) for security reasons. So you can not make the slave to be the master easily as the slave has not an exact copy of the master database, it has just the data that is relevant for its functions. My recommendation is that you restore the master server from your backup, the slave should work until then and serve the websites and mail services until the master is online again.
If you really want to make the slave a master, you will have to insert the missing data from the amster database to the slave database e.g. by restoring the mysql database backup of the ispconfig database of the master on the slave. But I did not tested that yet.