You have a couple of possibilities. If the grave is marked, there is a good chance it was indexed in a survey of cemeteries in Monroe County.
First, I would search at www.monroecountyhistory.org
, the website of Monroe County Local History Room in Sparta, Wisconsin. Fabulous staff & volunteers, a very strong cemetery index, and newspaper, photo, census, birth, death and marriage record indices, to get you started.
You may find more about your ancestor than just the confirmation of a marked grave in the cemetery!
Or try this link: http://www.usgwarchives.net/wi/monroe/m-cem.htm
and click on the Big Creek file link: BigCreek-cem.txt in the list of Monroe County cemeteries. There you will find the list of marked graves from the last cemetery survey.
If your ancestor is not listed it does not mean he was buried somewhere else. Some graves had markers of a more temporary nature (wood, etc.), as that was all the family could afford, and so the grave might be there but is unmarked.
The staff & volunteers at Monroe County Local History Room (200 W. Main St. Sparta) are a terrific source of information and should be able to help you.