Many theme installs replace the system menu, which also removes preloader. If thing go south during the install, recovery is difficult. Unfortunatly the best solution is a boot2 bootmii install, but your is an ios so we can not use it. Preloader is removed from the system, this is why it does not load anymore. Your only real hope of a soft-recovery is if you have a functional recovery menu via savemiifrii.
Give this a good read-
OH SH*T: Black Screen of WII!!!
Post back here with your findings.