What make and model modem?
Do you have the User Guide/Manual?
If not, go online to the manufacturer's website. Find and download the manual. There should be a section explaining the lights and what colors and flashing patterns indicate. There should also be a troubleshooting section.
And there may also be manufacturer's FAQs and a forum - search therein.
You can also use the manual to double-check your configuration and how you are connecting your network devices. Look for a diagram and verify the connections, cables, and ports being used.
Could be that the modem has a problem but you need to eliminate possible connection and/or configuration issues.