Launcher is an app which gives interface between user and other apps.We can delete default launcher by root access.If default launcher is deleted (Assuming no other launcher is installed on phone), then OS will work but we can't access any other apps from phone and it is looks like crashed phone.
So when you reboot without a launcher it will not go to any home page because there is no launcher at all. So
1> It may show a blank screen 2> It may show company symbol with sound 3> Some times it shows "system stopped working" message
In this situation if you factory reset the phone (using boot loader) then also you will not able to get the launcher because the default app in the ROM is deleted.
To solve this you have to install ROM again you can also take necessary data backup(Using custom recovery).
You can also switch on phone and install launcher from Play Store using laptop/computer.Go to Play Store in PC and hit install on any launcher and it will install on phone.(You have to connect your phone to internet and signed in from same Google account)
You can install launcher app using a zip file. Flashing a proper zip file (which contains launcher app) in custom recovery.