! symbol infront of project in eclipse even if there are no errors

I am using Eclipse Java EE IDE for开发者_运维技巧 Web Developers.

Version: Helios Release Build id: 20100617-1415

It gives me an ! mark in front of the package icon in the Package explorer at times all of a sudden even though there is no error in the files. I am unable to build the project. I tried clean up and build again but it's not working. Here is the snapshot of the Package explorer.

Have a look at the Problems panel or Markers panel to see what is reported.

Also your snapshot does not show.