If you get a stain on your cross stitch fabric, you should try to get it out - immediately. Be sure to read the laundering directions that came with the fabric first, but here are a few suggestions for dealing with stains.
Consider the type of stain before you treat it. If it is a blood stain, rinse the fabric with cool water. An ink stain - try hairspray. Cola - if the stain is fresh, a mild detergent should do the trick.
Also, consider the type of fabric. Tea-dyed linen will need to be treated a little differently than plain white Aida.
If the stain is stubborn and refuses to come out, then you have several options. If the piece is a small ornament, it may be best to start over with a new piece of fabric. If the stain is on a huge project you have been stitching on for months, then you may want to cover up the stain, depending on where it is on the fabric. Add a border, an additional motif, or a few extra stitches.