OK!
Let's start here:
Did you recently upgrade the Mac OS to Yosemite?
If so, did you installed the FTMM patch? You should have been alerted to do this the first time you opened FTMM after the Yosemite upgrade.
Are you syncing with ACOM (ancestry.com) on-line member trees?
This may sound too simple, but It may be a case of shutting down your machine, an actual shut down, not restart. Wait a couple of minutes and then restart your machine. Make sure you quit all open applications (and sign off of your ACOM account in FTMM) before shutting the machine down.
After your machine is rebooted and is on-line, fire FTMM up, and sign-on.
If that doesn't work, another fix-it and can't hurt, is to run a permissions repair on you machine. You do this by restarting your machine, when you hear the chime, hold down COMMAND and "R" keys until the grey screen appears, then let go and wait until the a menu appears on the desktop, choose "Disk Utility" and follow the directions to repair permissions.