Shopify checks two DNS records. If either is wrong, the domain stays unconnected and the store may show an SSL error. The records are edited where your domain's DNS lives, not in Shopify.
dig +short NS yourdomain.com. Edit records at that company. If it says Cloudflare, edit them in Cloudflare.@ must be 23.227.38.65. Delete any other A or AAAA records on @. Leftover records from an old host are the most common cause.www must be a CNAME to shops.myshopify.com. Not an A record, and not a forward.If it still will not connect, send me the domain and I will tell you which record is wrong.
Want the rest of your site checked after it is back? Run a free 40-point check at tinkeraudit.com.