The red light on Linksys router indicates that something is wrong with the internet connection. Basically, underlying such a condition will be one of the following: 

• Internet Down: This is the commonest cause. Check for other devices in your household which cannot connect to the internet at the same time. Also, check for any outages in your area through your ISP's site. 
• Modem Problem: Sometimes this happens due to malfunctioning of modem and router will not get internet signal because modem is not working properly. Switch off the modem from the source of electricity for at least half a minute, and then switch on again. 
• Router Configuration Problem: Another reason could be improper router settings so that sometimes it will break the internet. Reset the Router to Factory Settings and then configure it again per your ISP.