https://www.shopifystatus.com/incidents They are aware .... ours just started working again, so see if it's working now  

https://www.shopifystatus.com/incidents They're looking into it.

https://www.shopifystatus.com/incidents They're looking into it.

https://www.shopifystatus.com/incidents They're looking into it.

It's not just you, it's affected many many stores over the past hour.  Hoping its fixed asap 

There is other posts here with the same issue, out website included.  Checkout just bounces back to the home page.  I am hoping Shopify will fix this?  Not sure if we need to do anything but wait 