Hi Phil,
We apologize for any inconvenience this issue may cause, as the service health is green with your account, the problem you encountered can be caused any incorrect configurations had made recently.
To narrow down the issue and find the root cause of the problem, firstly, I would like to confirm the detailed issue scenarios on your side:
1) With the team site accessing issue, can others from your company accessing the team site at this point, or your team site cannot be accessed at all?
2) If your team site cannot be accessed some times, can the problem be specified to particular computers or particular users?
3) For the secured libraries, can them be accessed with any users in your company or cannot be accessed anymore?
For the team site accessing problem, please try the following steps to troubleshoot the problem:
1, If you are still facing the “An unexpected error has occurred” error, please try to clear the cache and cookies of your browser, and see if there is any improvement.
2, Beside click Team Site link on the top of Office 365 portal, please type your domain name (http://yourdomain.sharepoint.com/) in the browser, login and check if your team site can be accessed.
3, In addition, I suggest you check the Storage Metrics of your account: In Office 365 portal, click Admin on the top, click Manage team sites under Team sites and documents, click Go to top level site settings, click Storage Metrics under Site Collection Administration.
Any updates and captures of error messages are appreciated.
Thanks,
Evan Zhang