I just took my front cog, chain, and rear wheel off to clean them. I put everything back on and now when I lock my legs up to stop (resist the force of the pedals) or skid, my chain loses all it's tension and I can't ride. Something is wrong and I can't figure out what it is.

Does anyone have ideas? I have a feeling this may be a somewhat popular problem and I'm just being a n00b.