Maybe use the DVD installer to upgrade the current system to 15.0.
Yes, I realize that you already upgraded. But it seems that something went wrong with zypper and related functions. So using the DVD installer for an upgrade should be using a working zypper.
If a LEAP 15 DVD won’t work, maybe a LEAP 42.3 DVD.
Whichever you use, an “upgrade” should re-install that version of openSUSE which should repair your system, even if using very old packages.
After a successful repair, you can then update to bring your system up to date.
And, if you say you “crashed,” if there is a better description like if an error displayed or what your upgrade might have last tried to do (even taking a camera picture of your screen if necessary), or if your system froze might be helpful