Without any specific info about the project we can't tell you much. If it appears out of nowhere, it could well be that the project is genuinely damaged. Other than that the standard steps apply: Open the project with capslock enabled to prevent rtendering/ evaluation, check your language and font settings in AE and on your system. Perhaps it's just a fluke because you changed something or picked a font that doesn't support Unicode fully. Anything beyond that will require more info.