All of these questions and more are explained in detail in this article:
Please check the knowledge base before posting next time, it contains a lot of useful information.
It’s because you have configured WordPress to use pretty URLs, but have replaced the default WordPress .htaccess rules (which are necessary to make this work).
Please restore the default WordPress .htaccess rules to your .htaccess files to fix this:
Also, generally speaking, specific website functionality not working is always first and foremost your responsibility. Even if it is caused by a hosting problem, the burden is on you to investigate it and confirm that it’s not a code issue, configuration issue, or any other issue out of our control.