Quick answer: check four things first: is the service active and paid, does the domain still point to your hosting, did you change something just before it broke, and what error does the page show.
Checklist
- Service status: in your client area the service must be Active. Unpaid services are suspended automatically (see "Understanding Invoices and Renewals").
- Domain: make sure the domain has not expired and still points to your hosting.
- Error type: a white page or 500 error usually means a broken plugin, theme or code change. Undo the last change or restore a backup.
- SSL warning: see "Why is my SSL Certificate not working?".
- Try another network: use a mobile connection to see if it is only your local network or cache.
Contact support
If it is still down, open a ticket with the domain, the exact error text or a screenshot, and what you changed before it broke.