.meta files does not have valid GUID

[butterfire]

Hi everyone, after I installed the Sprite 2d package I suddenly have lots of errors saying
The .meta file Assets/Packages/System.Reflection.Emit.ILGeneration.4.3.0/lib/netstandard1.3/System.Reflection.Emit.ILGeneration.dll.meta does not have a valid GUID and its corresponding Asset file will be ignored. If this file is not malformed, please add a GUID, or delete the .meta file and it will be recreated correctly

I am not sure what happened. I did have trouble yesterday and had to restore my project, but since then I have completed several more lessons and everything worked fine.
Is there a way to regenerate all the .meta files? Should I just delete all of them like the error messages says?

[butterfire]

I clicked right clicked “Assets” and clicked Reimport. Now all the errors are gone, I think that fixed it and everything seems to be okay now.

[Brian_Trotter]

Sometimes when Unity imports a package, there can be conflicts or errors with the meta files. Usually a re-import or a restart of Unity does the trick, though sometimes it does not.