New answers tagged solution-package
I faced similar issue as a result of SharePoint 2013 farm solution deployment failing in between. If a solution deployment fails it causes the SharePoint 2013 to retract the installed package. However during retraction it just deleted the feature folders and feature xml files copied during solution deployment but since farm scoped features gets deployed on ...
Try navigate to "View" on the top menu, then "Other Windows" --> "Packaging Explorer", so you can check that your Package has not been damaged
Depending on where you store the files, you may not have access to them. Sandbox solutions do not have access to the local file system (see more restrictions here: http://msdn.microsoft.com/en-us/library/gg615454.aspx#BKMK_LowPrivilegeToken). You would have to store the files in a library to gain access to them.
Probably you should associate workflow with the list (Visual Studio does this step behind the scenes when you deploy your project from it, but you need it manually if you use wsp file).
.webpart files are XML. Open it with Notepad or your favorite XML editor. See Web Part Properties - part 3 - the .webpart file by Wictor Wilen for an example.
Top 50 recent answers are included