Hey Gareth,
I’m not sure what is covered by your statement “I haven’t changed any settings”; however, it sounds like you have changed the security level under Project ‘ProjectName’ > Security > Security Level. It would typically start asking for you log in if you change the security level from off to Prototype or Production.
To log in, you can use the MxAdmin credentials under Project ‘ProjectName’ > Security > Administrator (the tab right next to User roles). You can also switch it off again from Project ‘ProjectName’ > Security > Security Level.