When the project build fails, the Project Explorer displays the “error” sign over the project's root folder and over the folders and files where the issues were detected. The Problems view reports about the issues that have caused the build failure:

Click an entry in the Problems view. If the issue is detected in code, the editor area displays the problem file and marks the line of code where the error is found.
Build failures with multiple unresolved symbols in code can be caused by the incorrect C/C++ indexer settings. Find the details in Adjusting the C/C++ indexer settings for large files.
After resolving all build issues, right-click the project in the Project Explorer and click Clean Project on the context menu.