How can I restore the previous system state after formatting my Macintosh HD?
First, recovering your files may be impossible. You should immediately stop using the disk. The data may still be there, but marked as free for storing other files, so the longer you use the disk, the more likely it is that something will get overwritten by chance. There are services you can buy that will try to recover what's on your disk.

Once you get that sorted out, regarding the boot problem:

According to this Apple Support article, it means the computer couldn't find a valid system folder to boot from. Try starting in Recovery Mode by holding ⌘R.