Error CS0103 UnityEngine does not exist

When I’m creating a new script it can’t find UnityEngine, so I can’t use all it’s classes. But old scripts in the same directory are working correctly. I tried to reimport project but it didn’t help.
This topic didn’t help me as well:

There is no more “Project > Assembly CSharp Options > Build > General” in MonoDevelop.
I have updated unity two days ago. Could it be the reason? How to fix this?

Try doing a reimport all. Sometimes editor bugs pop up, and a reimport will fix them.

Check your .NET framework version.

UPD: I forgot to say that I was using MonoDevelop. Today I re-installed Unity and instead of MonoDevelop it runs Visula Studio. How to change it back to MonoDevelop (because it launches faster than studio) ?
“Edit > Preferences > External Tools > External Script Editor” shows me only “Open by file extension” and “Browse…”.

Moving project folder to another directory, opening it and moving it back somehow helped me. Also thanks God I have an older version of Unity installed on my tablet, so I could copy MonoDevelop to Unity folder on my PC again.