The most common mistake when you sit online and want to reach your website is HTTP error 500.
If the website you created is yours and you just are doing websites for fun its ok, but if you are a small business and get this error then you lose money and the first intention for what you create your website is not achieving the main goal.
I highly suggest you transfer your website from your host immediately to new hosting or my preferred one Exai which is best website builder for small business. This is an awesome professional service which offers you for maybe 5 times less money fully managing your website according to your needs.
You can check also this link