The cpu and uart were working fine before, but one day the board suddenly didn't work.

Certainly any software changes that have been make between when it was working to when it stopped working should be considered suspect.

Presuming you have eliminated software changes as the cause, you might check to see if you have the auto-CTS# hardware flow control function enabled and whether CTS# is asserted. When auto-CTS# is enabled, the transmitter circuitry checks CTS# before sending a byte.

Check similarly if you have the device's software flow control options enabled.