Hi Lee Spottiswood thanks for reaching out. From the description i understand that you have a timer trigger that is set to trigger at 1AM only on every Monday's but it suddenly stopped on 14th aug and it got triggered again when you open azure portal.
Is your function app running in consumption or APP service plan? If it's in an App Service Plan, you need to make sure that Always On is enabled. It is enabled by default on Function Apps, but it's good to double check.
If it is on consumption plan please verify the steps mentioned in below document.
You can also run the Diagnose and solve problems on your function app and search for "Timer Trigger Issue Analysis" to get the insights and recommendation.
please let me know incase of further questions.