"Not a table" means more serious problem than TABLEVALIDATE can fix obviously. Also when DBF recovery fails the problem can be critical (if you have the DBF recovery which supports your DBF file format).
You should look at the DBF header via some hexa editor. You may even use the one from VFP:
DO HOME() + "Tools\HexEdit\HexEdit.app"
It will ask for a file opening so please select your DBF and post first two lines here or look at the DBF structure description in the VFP help.
First two lines (in hex editor) contain the file type, number of records, record size, last update etc. "Not a table" means the first byte was not recognized as valid DBF file marker.
Additional lines contain column descriptions each one in 16 bytes.
More info: http://msdn.microsoft.com/en-us/library/st4a0s68(v=vs.80).aspx
Second easiest fix is use DBF Recovery Toolbox or some other applications. Search in Google. http://www.oemailrecovery.com/dbf_recovery.html