Hi Paul - sorry you're seeing an issue getting Pro started. First things first, a question like this would be best asked in the ArcGIS Pro space of GeoNet.
With an issue like this, there are a lot of different things that could be going on, meaning a lot of questions and troubleshooting, so to be honest the best path forward is probably by working with technical support: Esri Support Contact Support
But to get started:
1. What version of ArcGIS Pro do you have installed?
2. Is it installed on a local machine (what kind of machine - laptop, desktop, specs) or are you accessing it remotely?
3. Have you had another version of ArcGIS Pro installed on the machine previously? What version?
4. When did the initializing problem start? Had you been successfully using Pro before that, and for how long? What changed?
5. If you had already been using Pro, had you added any Portal connections beyond the default of arcgis.com?
6. How are you authorizing Pro - Named User (through an ArcGIS Online account or ArcGIS Enterprise?), Single Use, or Concurrent Use?
7. Did you make any changes to Python environments?
8. Do you have any add-ins installed? (e.g. ArcGIS Solutions Deployment Tool | ArcGIS Solutions )
If so, one thing you could try when launching Pro would be to temporarily disable add-ins: Manage add-ins—ArcGIS Pro | Documentation
Hope this helps in the troubleshooting process!
