Cisco routers are very reliable but no router is perfect. Routers can go down at anytime due to bad hardware, failed IOS upgrades, or just plain human errror.
Your default gateway router is your connection to the outside world (that’s the Internet). I recommend you configure the hot standby router protocol (HSRP) on your Cisco roger functioning as your default gateway (perhaps other core routers too).
To learn all about HSRP and how to configure it, read my article: Ensure Cisco router redundancy with HSRP