Based on what you describe I suspect an issue with the I2C bus.
Suggested tests would be either to
a) Comment out the wire.xxxx call lines in the si5351 tab or
b) Load my test program (in the file section) as it performs 3 gradual tests of the bus, including writing to the si5351 but with a timeout. So it will always return a result, positive or negative.
From memory the wire library uses blocking call for sending data over the bus. So if the communication is not possible it would hang there.
Reasons could be defective nano A4/A5, short/open circuit in bus lines, defective si5351.
73, John (VK2ETA)