How to get back my beautiful crystals made from iron and sulphuric acid
I realize this question is over two years old, but I have to add the correct answer here: the iron (II) sulfate you had at the beginning was in the heptahydrate form.  SEVEN molecules of water for every iron (II) sulfate ion pair.  Now, what do you think would happen to that crystal structure when you add one of the most powerful dehydrating agents in existence?  That's right, it sucks all the water out of your crystals, and you're left with a powdery, noncrystalline form.  You could have just added less concentrated acid, that was diluted enough to not be such a strong dehydrating agent.

Incidentally, iron is a surprisingly strong reducer of sulfates.  I did this reaction recently and noticed gold flecks floating around in the solution.  I was shocked to see gold, since I obviously never added any, until I realized that the iron had reduced the sulfate all the way down to sulfide, and what I was seeing was iron pyrite -- fool's gold.  Adding more acid eliminated that almost immediately.