My watch suddenly switched to a full circle form. How do I change it back to digital?

Try this;

First try resetting your Watch. Press and hold the Digital Crown and power button together until you see the Apple logo, then release. The Watch will reboot. If that doesn't work, you can go to the Watch app on the iPhone and set your Watch face again. Or, you can swipe the Watch face left or right to show what Watch faces you have configured on your Watch.