roll back

Unless you ran the DELETE inside a transaction, allowing a ROLLBACK statement to undo the action, it will not be easy.

The next best method would be to restore a copy of the database from backup, and reload the table with an INSERT/SELECT statement.  If the database is in full recovery mode you should be able to restore it to a point in time just before the DELETE was run.

If you don't have usable backups, you might be able to use third-party log reading tool.  I am not sure how optimistic to be, because if your database is not in FULL RECOVERY mode the logs may already have been overwritten.  If the database is in FULL RECOVERY mode, why wouldn't you have usable backups?

+1 to Scott C's response.

The only other hope you could have is having the data on a reporting system that you could access from the previous day.

Are you using Windows Volume Shadow Copy?  If so, you might have another form of a backup.

As a final option, if the data is on a SAN then I would also check to see if some sort of volume shadow copy technology is in use to see if you can access the data at a point in time earlier in the day.  This is similar to the previous option.

In terms of prevention, be sure to review your overall security to prevent an issue like this from occurring in the future.

Thank you,
Jeremy Kadlec