This error happens because your site has exceeded the limit of usage of server resources. This problem may be happening because you are loading your files, and then looking to see if your site is loaded. This practice generates a lot of traffic for just a visitor. You can try to upload many files before cchecking, or use a proxy service to access your site as another visitor. This error will also happen if your site gets a lot of visitors or because you are using a very resource-hungry script.
You can refer to our other article Is there a way to remove the "CPU Limit Exceeded" error?.