everything became pink

If something in your scene is pink it is the graphics card's way of telling you it doesn't know which shader to apply your object. In other words your objects are missing their materials.

Click on a mesh that is pink, and you'll see the material property will be set to none or missing. You'll need to remap the materials in your project to your pink objects, or simply drag the textures back onto your objects again (which will automatically create and assign a material to that object.

Are you sure all your materials are still there? I had a similar problem once and it turned out all my materials had just been removed from the game objects

i think your game is ok.
if it is showing pink means u have deleted some thing from the project.so it is not getting the reference of the objects so it is displaying pink . check once if u have deleted anything from the project

Change "Material" setting of the terrain to "Built in Legacy Diffuse" - that fixed it for me!

Importing an old (2.x project ?) into 3.x editor, things might get corrupted and missing. Most of the time is the missing material you see above.

Try to export assets to packages in 2.x editor and then import them in 3.x, it sometimes helps. From what I aware, there are no universal solution either.