I Got a MBP 2009 still with DVD I installed High sierra 10.13 with the (not officially supprted but MacOS Patcher made it possible) And encrypted SSD which my Company's policy demand you to do that if you want to receive email.
one day it just freezed I tried to rebooted it and found the system can't found wifi adapter anymore. reboot again same. reboot with command R found no recovery is there? may be when you encrypite you HDD and the recovery partition is removed?
reboot again system can't boot anymore shows a Prohibited sign (no entry / no parking sign)
I thought reinstall might work. I made a USB of officially supported sierra can't boot same Prohibited sign
reset VRAM? reset CRM? doesn't work. command+R won't lead you to recovery since the HDD is encrypted? I format the HDD in another mac. now shows and quesiton make sign when boot from HDD. if I boot from USB still Prohibited sign, but the same usb can boot in another working mac.