crontab -r removes the only file containing the cron jobs.
So if you did not make a backup, your only recovery options are:
On RedHat/CentOS, if your jobs have been triggered before, you can find the cron log in /var/log/cron. The file will help you rewrite the jobs again.
Another option is to recover the file using a file recovery tool. This is less likely to be successful though, since the system partition is usually a busy one and corresponding sectors probably have already been overwritten.