If the end user is using OSS, they only get 10,000 build credits - which means, the user will run into this, where the event request will be rejected and then on occasion will look at their OSS credits and see they’ve used the 10,000 allotment. This is in the plans/pricing area, and looks like this:
If the user is building open source and happen to spill over the OSS allotment, have the user email the Travis CI Support Team. With info about their VCS, and how many credits (build minutes) they would like to request (should they run out of credits again you can repeat the process to request more or to discuss a renewable amount).
I’ll retype what I typed in my original answer, maybe you missed the snippet about how the admins receive notifications about their OSS allotments going over.
(There’s also a huge banner that invokes this message if 10,000 credits are used in an OSS project).
As an external contributor you probably can make a good assumption given context clues, one of the clues you gave out is “The builds stopped suddenly”. That at least told me without even having to check (although I did) it was most likely a credit issue, another example is, if there’s a working .travis.yml, and it’s a OSS project and nothing is firing, there’s a good chance it reverts back to a credit issue.
Asking here is not the only way, the most obvious and logical other way is you could ask the repository owner, see if they have ran out of credits or disabled Travis on their repo. There’s also support@travis-ci.com.