Chances are your device is configured to only use itself for DNS lookups, which is quite common with pihole setups, and now pihole isn't running then resolution is failing.
If you can ping 8.8.8.8 as Dan suggests then just add another nameserver line to /etc/resolv.conf (note - this temp change should get you back up and running for you to get pihole installed)
sudo nano /etc/resolv.conf
nameserver 8.8.8.8
once you add this line and save the file, DNS resolution should work again when it realises your localhost name server is down.
Edit: ah, posted too soon. If you want to install the beta from scratch you can use the steps here Installation beta - #3 by DL6ER