What would cause the loss of coolant following timing belt failure?
The water pump is a prime suspect. Was the water pump replaced with the timing belt? If it wasn’t, it may have been leaking before, though not terribly noticeably. That is a common cause of timing belt failure. If the water pump was replaced, it is possible the gasket or seal were damaged or not properly installed. Take it back to where you had the belt done and have them pressure check your cooling system to determine the cause of the leak/loss of coolant.
I don’t think that the OP had this problem after timing belt replacement, but after timing belt failure.
Either way, the water pump probably is the prime suspect. Breathe a big sigh of relief since this engine isn’t an interference engine, so the timing belt failure probably did not wreck the engine. When ever a new belt is installed, a new water pump should go on anyway, and whenever the work is done the cooling system should be checked for leaks.