Earlier in 2014, Microsoft announced that custom code in sandbox solutions would be deprecated https://blogs.msdn.microsoft.com/sharepointdev/2014/01/14/deprecation-of-custom-code-in-sandboxed-solutions/. On Monday, 29 July, 2016, Microsoft announced that it would be withdrawing support for code-based sandbox solutions in SharePoint Online and Office 365. New code-based sandbox solutions cannot be deployed and activated in SharePoint Online and Office 365 as of now, and existing sandbox solutions will stop working in the course of the next few weeks. This will pose a serious problem to the thousands of customers using sandbox solutions in their SharePoint online, since their solutions will just stop working.
Microsoft’s official announcement about switching off the sandbox solutions: http://dev.office.com/blogs/removing-code-based-sandbox-solutions-in-sharepoint-online
Recently, while working on SharePoint Online project, I faced the same issue when my sand-boxed wsp stopped working and other sandbox wsp’s were not activated. So, hurry up and change your sandbox solutions to AddIns, as suggested by MS.