Your Let’s Encrypt client software should be able to log or display the specific error messages that help you understand exactly what’s happened.
Either way, you can consult
to understand the different rate limits that apply to different kinds of use of the Let’s Encrypt service. All of these except for one reset automatically after a period of time, so once you understand which rate limit you’ve hit, you should be able to predict when you’ll be able to use the service again.
The exception is “ridiculously excessive traffic”; if you’ve hit that one, you have to contact Let’s Encrypt staff (which can be done through this forum). Again, your Let’s Encrypt client log files should indicate the exact rate limit error condition that applies to you.