Script, this is a known bug that has been reported before. You still have all your Friends; you can check that on your own Profile page where you can see all of their avatars as usual, along with the correct Friends number. The bug is that the Friends list in your left-hand panel on webpages fails to load the data and shows 0 Friends & 0 Invitations, although you may have dozens of Friends & even some unanswered Invitations at the moment.
To fix the bug, it's usually enough to launch the game window; this usually "kickstarts" the left-hand panel on webpages, too.
Another, separate Friends bug, is the one mentioned by Snaike. You currently have no way of knowing if someone deleted you from their Friends list, because those people would still remain on your Friends list. This is a bug that WGTalex said will be fixed so that friendships really become mutual at all times, as they should.