For years SSRS was part of SQL installation. With SQL 2017, SSRS is no more bundled with SQL server installation. SSRS is now a separate download and install.
After you have installed and configured SSRS and it was working, a few days later if you received SSRS webpage failing to load, first check the license. You were probably running SSRS in evaluation mode, and that has timed out and the license has expired.
Solution install SSRS key
How to install SSRS Key?
1. Run SQL Server 2017 setup and copy the pre-populated key:
2. Run SQL Reporting Services setup chose enter the product key and paste the key that was copied:
3. Click next to upgrade SQL Reporting Services:
You should be back in business and ready to go.