I have been building my Android application for 2 years and I have never once had this problem until now.
When I load Eclipse, there is a little progress bar in the
The First thing I do is
make sure you save your project and all it's resources in a different location to keep it safe if something goes wrong, this is a good idea in general (personally i use drop box so that even if something gets worse i can still recover it)
Try closing eclipse and starting again, if you get the chance try it while plugged into your router with a Ethernet wire.
Make sure to check that Eclipse is up to date so that there are definitively no conflicts.
If this doesn't work try un-installing and re-installing eclipse.
good luck.