Train schedule and travel time from Guangzhou to Lianyungang China.

Well, no direct train runs between these two places.
So you are suggested to take bullet train from Guangzhou South Railway Station to Xuzhou East Railway Station first. The schedules are 06:47 and 08:11. The duration is nearly 9 hours.
Then take a taxi to Xuzhou Railway Station. Transfer to the normal speed train to go to the destination. The trains depart at 03:44, 04:57, 05:42, 06:34, 07:00, 11:23, 11:44, 13:48, 13:58, 16:32, 18:56 and 20:39. It takes around 3 hours on the way.

how much does this cost? can you travel over night?

From Guangzhou to Xuzhou, the second-class costs CNY 699.5; the first-class costs CNY 1088.5; the business class costs CNY 2193.5.
From Xuzhou to LYG, the hard seat costs CNY 29.5 or 32.5; the hard sleeper costs CNY 75.5 or 78.5; the soft sleeper costs CNY 115.5.
Well, you can also take an ordinary train from Guangzhou to Xuzhou, but it takes about 21 to 26 hours. There are four departure daily at 8:00, 9:12, 21:00, and 23:05. You have to spend a whole day and night on the train, not just a night. You need to pay CNY 206 to 253 for a hard seat; CNY 353 to 424 for a hard sleeper; CNY 640 to 665 for a soft sleeper.
Or, you may take a direct flight to the city (about 2.5 to 3.5 hours). It is more convenient for you! :)