Red X on the left side of the android project, but no error message is displayed in the project.

Don't be confused. If both clean and restart are ineffective,

In this case, you need to view the problems view. The shortcut key is Alt + Shift + Q, X.

Search for the Problem description and find the answer.

If the problem is described as follows:
"Error generating final archive: Debug certificate expired on 12/04/17"
This indicates that the debug certificate has expired.
Cause analysis:
Android requires that all programs have signatures; otherwise, the program will not be installed. During our development process, ADT uses debug keystore and sets it in preference-> Android-> buid. By default, the debug keystore is valid for one year. If you finish the android program from a year ago, the debug keystore may expire when you import the app one year later, causing you to fail to generate
APK file.
Now you only need to delete the debug keystore, and the system will generate a private key valid for one year for you.
 
Solution:
Go to debug. keystore and ddms. cfg in the C: \ Documents and Settings \ Administrator \. Android deletion path.
(Directories in different environments may be slightly different. You can find this path in Eclipse: window-> preferences-> Android-> default debug keystore under build)
Then re-import.