2d side scrolling sprites turn solid magenta

URGENT - New to Unity and a 3d beginner. i’m creating a prototype and successfully imported sprites. they have all turned solid magenta in the scene and game previewer (assets are fine and preview ok in the assets window). i can redo the environment by placing all the assets again and they render fine, but this is the 2nd time it’s happened.

are there size limitations for 2d sprites? I’m following tutorials but using my own art - sprites .png’s for transparency
I need a permanent fix. please explain solutions step-by-step/carefully.
I have a Mac:
processor: 3.5 GHz Intel Core i5
memory: 32 GB 1600 MHz DDR3
graphics: AMD Radeon R9 M290X 2048 MB
thank you for any help

I found that by hitting reset the sprites turned from solid magenta to their .png original state. sprites still turn magenta but by hitting reset they revert. however. now i can’t see the game preview. it’s blank. seems like a bug in version 5.4.0b17. Sprites keep turning magenta