vWork only allows you to use the same username and password once per session (or computer).
If you log in on a new computer, or a different browser, using the same login credentials and don't log out of the earlier sessions you are automatically logged out of the earlier sessions.
You can be logged in to the mobile app and web app at the same time using the same login credentials.
If you are concerned that you may have been mistakenly logged out, you can reset your vWork password. You can find the steps to reset your password in our article, How do I reset my password?