This is probable a very stupid question, so please don not shoot me down in flames.
I have setup Pi-hole on my pi 3B+ and given Pi-hole server the ip 192.168.1.72 which out side DHCP server assigns IPv4 addresses from 192.168,1.100 to 192.168.1.200 and set the Preferred DNSv4 server 192.168.1.72 and Alternative DNSv4 server 192.168.1.72 and all is working well.
My first device to be connected to Pi-hole is my laptop which is running Linux Debian 12.6 (bookworm) and in the connections system settings I have changed the DNS Servers to 192.168.1.72 so that it see my Pi-hole and all working just fine.
Hear comes the stupid question if the Pi-hole go off line (like switched off), is there any way to get back to the internet or do need to re-configure the router and connections system settings on my laptop to re-establish the internet connection.
Pi-hole should be the sole DNS server for your clients (no others or ads will leak through)!
So yes if Pi-hole goes down, you'll have to reconfigure your router LAN DHCP DNS settings to restore Internet connectivity.
You'd also have to let the clients renew their DHCP leases to receive the newly configured DNS server(s).
Or if have set DNS on that Debian laptop manually, you'd have to configure another DNS server(s).
I have two Pi-hole nodes on two different Raspberry Pi's for my home network to prevent disruption when one goes down.
For example when goes down while upgrading, tweaking or making mistakes.
Heck I've even got five Pi-hole instances bc I'm also experimenting/learning a bit:
one "primary" on a Pi 1B;
one "secondary" on a Pi 1B+;
one running in Docker with stable v5 on my NAS;
one running in Docker with beta v6 on my NAS;
and one running in a VM with beta v6 on an old Intel NUC that I turned into a hypervisor;
There are many options to create a second Pi-hole node for redundancy.