If none of the other suggestions work and you want to try to avoid a trip to the store, hard reset your phone with the keys.
Please note that all data will be deleted from the HTC EVO 4G as a result of this procedure. This process does NOT delete data stored on the microSD card.
If you selected YES, all data including third-party applications and personal info will be deleted from the HTC EVO 4G. Once the wipe is complete, the phone will reboot to its factory fresh state.
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.