I am sorry to hear that you are having some issues.
Before moving to Cloudflare, was your Website working over HTTPS connection?
If so, did you had an valid SSL certificate installed at your origin host / server which covers both your naked (root) domain any any other needed sub-domain like www, mail, etc.?
Regarding Cloudflare 520 error, may I suggest you to try looking into below articles to troubleshoot the issue:
Due to the SSL, may I ask what SSL option have you got selected under the SSL/TLS tab at Cloudflare dashboard for your domain ( Flexible, Full, Full Strict … )?
Here is a way to re-check if you correctly setup the SSL for your domain with Cloudflare:
Regarding available SSL options at Cloudflare dashboard, check here:
If any other issues appear, follow the needed steps for troubleshooting from article below:
In case you do not have an SSL certificate, you can use Cloudflare SSL, if so, kindly make sure you follow the instructions as follows on the below article to setup an SSL certificate using Cloudflare CA Origin Certificate: