As a passenger waiting at City Thameslink....it was diabolical service and communications.
The problem wasn't the Cambridge train. The issue was north of City Thameslink. In terms of the first trains blocked to my knowledge, a West Hampstead was taken out of service into I think the Snow Hill sidings. The Cambridge train was cancelled when it was at City Thameslink (and then uncanceled - see below). But while it said this on the screen, the train and the driver didn't seem to know this and it was around half an hour of doing nothing.
There were no announcements on platforms to customers. Station staff were entirely ineffective - providing mixed messages and unhelpful advice. In their very limited defence, it was also very clear there were getting almost zero comms from any central function and there was some quite hopeless incident management and communication going on higher up.
The problem was essentially no electrics on the northbound from City Tx to an unspecified point (i.e. at least Farringdon, possibly further). Loads of southbound trains went through. After what seemed like an hour, eventually these were paused and northbound trains were run up the southbound tracks.
However, from a City Tx perspective - this was not announced to customers. The station manager mentioned it as a vague possibility after we'd managed to summon him from his office, but that even if it did happen the trains wouldn't stop at City Tx and the doors wouldn't open, not least at the station was meant to be closed by this point). Were were eventually advised to get on the Cambridge train which would be reversing back to Blackfriars. (The Cambridge driver was good at comms within the constraints).
I think the data is probably just very confused - in terms of platforms through the core, it went Blackfriars (North), City (North), Blackfriars (South), City (South), Farringdon (South), St P (South). It was back to normal for the subsequent stations.
While still speaking...a Bedford bound train appeared on the 'wrong' platform at City and lo-and-behold opened its doors. However, no one could get on due to being on the wrong platform. I don't think any of us took the risk of changing platforms in case another appeared - again, not a single announcement from staff on this.
Another one or two Bedford trains rapidly appeared (again, we were on wrong platform...).
In terms of my personal journey, I got the Cambridge train back to Blackfriars. That waited for a while then went North. I changed at St P for a Bedford train. Unfortunately, in GTR's infinite wisdom - the calling pattern didn't change. So this was a slow "all stations" train (staff still call them all stations) but didn't stop at Harpenden, Leagrave, Flitwick or Harlington. So a group of very tired passengers, buggies, and kids, had to change in Luton. To our great relief (and a huge amount of luck at this point), this only required a 5 minute wait and no change of platform.
In contrast, the lady on the St Pancras platforms was incredibly good although she also lacked information from above.