Shake it off and start again. We all have good and bad days. If you repeatedly can’t get past 48 hours, then look more seriously at your routine. But otherwise, let it go … and start again.
EDIT: Edit to say that it’s never easy on your first fast. Do a couple of 48 hour fasts, or 36 hour fasts, or 24 hour fasts, and work your way up to longer fasts. If you have a setback, just accept it, and start again.