Here is a link to the service manual for your desktop.
Scroll to p.77 to find the part numbers for your particular motherboard (90006404 for MB FRU W8P Lenovo Part No. 14116).
If you search online using the number 90006404 only in the search term you will get results for suppliers of the motherboard.
Here is a link to just one supplier of the motherboard. It is not a recommendation to use them. It is shown to give an idea of the cost of the part. There are other suppliers (not many) online that may suit you better.
Hopefully this is of some help, if you cannot repair your motherboard.