How are you updating the channel (embedded device? browser? Something else?) If it is an embedded device, have you checked if it has internet connectivity? Maybe it lost internet connectivity?
You can debug the problem like this:
1) Create a new ThingSpeak channel
2) Go to the API Keys tab and oyu will see on the right a section that says "Write a channel feed". Copy the URL and paste it into the browser. The URL should be something like "https://api.thingspeak.com/update?api_key=__YOUR_CHANNEL_API_KEY___&field1=0"
Now confirm that field 1 of the channel is updated by going back to the channel's private view. If this is the case, your ThingSpeak account has capacity and you have demonstrated that you can update channels.
Then I would focus on the device that is updating the channel. If it is connected by serial port, perhaps you can put some debug code that will print out the status code you get when your code updates the ThingSpeak channel.
If you are still having trouble, share the code on the embedded device (redact the API keys) and the status code you get when updating the ThingSpeak channel.