This happened to me and was really an unpleasant surprise. I was up late and plowed and planted a 20x20 field of Sweet Seeds for Haiti. This is a six hour crop so I was planning to harvest it in the morning. When I got up in the morning and opened Farmville there was just an empty plowed field. What's going on here!
Well I realized that sometimes my internet connection gets lost. Since the state of you farmville game is stored on the server, if the connection is lost for a period of time, even though the application is running locally the information does not get to the server. Did you ever get the big cow message "The bits got lost on the way to the server"?
If you re-establish the connection before closing the browser everything usually everything is OK and you will find a nice field of sweet corn in the morning.
So how do I know if what I did just made it to the server?
The best way I have found to be sure is to go visit a neighbors farm before quitting. If you don't get there, i.e. the screen just stays on your farm, then your connection is lost. You need to re-establish your internet or wireless connection and try again. If you do get there, you can help them and come back home. When you get back home you will see your seeds are planted. You can also click on the free gifts menu at the top.