If you don't care about the data, re-run the FreeBSD installer, select a "custom" installation type, select option 3 ("Partition") and create a new partition (you can press
A to have the partitioning tool grab the whole disk, then just change the type to something Windows will like (7 = NTFS) and Windows should be able to see it & format it).
Remember to write the new partition table (
W) before exiting.
Obviously be extra careful, since a mistake here (like selecting the wrong drive to re-partition) could really ruin your whole day...
Obviously you should be able to do the same thing with the Windows partitioning tools (but I don't have any of those handy).
If you want to actually get your data back the answer is somewhat more involved (You'd need to re-create the partition table as it was before, which presupposes you know the old layout. If not you start getting into disk exploration or recovery tools).