Holly,
That link still works on my side, both from inside and outside our own network so I think we may need to identify a possible issue with the email itself. Just to cover the bases completely, have you (or your IT team) recently made any DNS or CName changes for your instance? Was the Branded Tracking Link feature recently changed/enabled?
Those questions may require you to reach out to your IT team, just to ensure that things weren't changed without your knowledge.
Additionally, I just tested the link on mobile devices and it is operational there as well.
At this point I'd recommend filing a support ticket (support@marketo.com or via the Community Support Portal) so that we can take a look at the actual email to see if there might be something wrong there.
- Jesse