I've never encountered this issue before, not sure how this happened other than perhaps the Config file is corrupted.
You could try going into the BIOS, boot options and add a new boot option, point it to EFi/OC/OpenCore.efi, give it whatever name you like.
Move it to the top, save and reboot.
At boot, press F12 so you can select the new boot option, it should be the 1st boot option, but this is to be sure it boots to it.
As a word of advise, always have a backup before making any changes to your EFI folder.
Either have a bootable USB you can edit for testing or make changes on the internal EFI but have a working USB handy in case of boot issue like you're having now.
The USB don't have to have an installer on it, just need the OC EFI files.