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Were you experiencing dropouts or the internet was not working at all?
How are you connected to the modem? Is it via WiFi only or both WiFi and Wired connection?
Have you tried to reset the modem/router while it's on?
BasilDV
If the devices are connected via WiFi, then signal interference might be causing this as well.
You can troubleshoot with the help of below articles.
You can also perform a factory reset to the modem/router if the issue will recur after the troubleshooting above.
Let us know how it goes.