There is one method that will get you time to research. Enter Recovery.
I have been thinking about another solution. I can not recommend this, because I do not know for sure what will happen, but if this happens to me I will try to check if the system installer show up as a disk if I follow this Startup Disk And then if it does, I will choose the disk I have my old system on. If it does not work, I will do a hard shutdown at once. Never if it has already done one of the reboots the installer always does.