By 'stopped working', I mean that the prompt 'no rows exist yet - create the first one' appears, but clicking on it does nothing. Same for the '+' symbol - clicking on it does nothing.
Everything was working up to this point, and I can still add values to other fields for that Channel. I was able to enter values into columns in this particular matrix fields until recently, plus select entries in other columns using the Playa fields. The templates were also successfully checking the Matrix column values.
The Matrix field includes 3 Playa field types and 2 text. Have I just overloaded it?
Also - the site is in development. It's a friend's website and I'm not familiar with the web host. The site is running quite slowly, and there may be a problem with intermittent IP blocking. Could that kind of thing cause just one Channel field (the last field) to not respond?