Failed To load Catalina

Was running Mojave.latest on MacBook Pro and saw Catalina was available in Settings so decided to load it. Since it was taking some time, I stepped away and came back to an error message — not enough disc space on Hard drive. Can’t reinstall Mojave, can’t get back to hard drive to try deleting to create space — I’m stuck, and I’m an online professor who needs to get work done. Can anyone please help?