I tried to update my Mac Air (2013) with the latest OS Catalina update. It already had Catalina installed successfully. So this appeared to be a minor update, security, or fix type update. I didn’t read closely. 
I came back to check on it and it told me to quit Mac OS installer bc I didn’t have enough free space. It asked me to select a disk to boot from. There was only 1 option…my hard disk. When I did this it never came back. I only get the logo for a second and then nothing. I was able to get to Disk Utility… But haven’t been able to get a boot up since. Any ideas for me?
I apologize if this isn’t exactly on topic…