Progress Has Gone Missing.
Unfortunately sometimes the game experiences rollbacks, when game progress either doesn't save correctly or needs to be put back to a previous state in the case of a big glitch etc.
Unfortunately there is no cure for Rollbacks, if it happens once, the best advice is to grin and bear it and use it as an opportunity for more XP.
If it happens repeatedly, Contact Zynga.
Please remember we're not Zynga, these are simply steps we've devised over the months of playing that may help solve some of the in game issues.
If you have something that isn't covered here, please come and ask us a question on our Facebook Page.