After installing elementary OS the system ran fine at first, but after I restarted my computer today the top menu bar called WingPanel has gone.
Is there any way to get it back?
You should be able to uninstall it by typing (in Terminal):
sudo apt-get remove indicator-synapse
Restart and check if it has any effect.
If yes and you added a PPA just for this, you should also remove the PPA.