Installed security update on 10.13- won't boot anymore

As the title says: it won‘t boot and I tried the usual stuff that works (disabling nvda, sip etc) The bar loads to 3/4 and then reboots. Halp. Havent got an install stick handy. I'm a lil nervous the update wrecked my system tbh. What do? Everything worked fine in 10.13 before this little update