This is very frustrating to see that the app working fine yesterday is not working today. There might be some problems with your Work space or Project Configurations. This tutorial will show how to solve it without wasting your much of your time.
Go To Project (Top Menu Bar) –> Clean Your Project. That Might solve your problem as cleaning the project will rebuild your entire project including your resources (R.java) file.
Solution – 2
Right Click On Project –> Properties –> Java Build Path. Make sure you have all the jars there which your project is dependent on. (Also Have jre library and android.jar also).
If Not, Go To Libraries & import hem.
Also remove the jars that have imported before & does not exist now. Like this, Remove jar with red cross mark sign or import them again from correct location.
Solution – 3
Sometimes It may be because of your old ADT plug in. You can update it to new version. Go To Help –> Install New Software –> Add new software if you have already downloaded new ADT.
Go To C:usersYour UserName.android and delete debug.keystore file. Again Restart the eclipse & rebuild or clean the project.
Make sure that your project don’ have dependency in another project in workspace. If yes, make sure that project’s don’t have errors or unrelated dependencies.
Make sure There is no error in your AndroidManifest.xml file as your project starts running from that point also.
If All Above 6 solutions Don’t work then create new work space –> Create new project & copy older source files & libraries here, Configure build dependencies & rebuild your project. Now It will work.