I have updated my os to high Sierra yesterday. Since update unity3d is not working. The problem is when I want to open an existing project then unity acts as it's doing something but nothing is happening. The project never opens. I tried to reinstall unity but it didn't help. Is there any way to solve this problem excepts downgrade to Mac OS Sierra?
You can try to read the official post on this topic. The possible solution for the trouble like this is:
we were able to track down missing Project View items issue as APFS and Unity editor interoperability problem. We are working on a fix, though meanwhile we recommend to not upgrade your main filesystem to APFS or at least keep Unity editor and your projects on HFS+ partition.