My project worked as first. Now when i try to run it it says "unable to find Ant file to run" (or other Ant-related error messages).
This can happen when you for instance move the Eclipse installation folder, or the Workspace (= folder where Eclipse stores your projects). But there are other possible reasons.
It usually means that your Eclipse project is broken.
Close Eclipse and then start it again. Then try to open the project. If it cannot find the project, try to import it via File > Import, General, Existing projects... (browse to the folder that contains the 'broken' project).
If that also does not work, create a new project in Eclipse. Then find the java files of the broken project (use Windows Explorer of Finder). Copy the java files and paste them in Eclipse (right-click the src folder, then choose Paste).