You overexerted your body and need to rest. If you possess Fatigue in excess of your Fortitude, you gain the Exhausted Status.
Recovery
Recovering from Fatigue simply requires resting. Assuming a good night’s sleep, you automatically recover from one instance of Fatigue each morning. If you rest for a full day, engaging in no strenuous actions, you can recover additional Fatigue equal to half your Fortitude with a successful Check (UC Fortitude, DT 10 + total Fatigue).