Your fork shouldn't be able to just slide in, in fact it's very very hard to push it in. What I think happend was you got a headset that was to big, that was my problem. My suggestion is bring your bike (with the fork) to a bike shop and have them find a headset that will fit. Because like I said, your fork shouldn't just slide in. No worries though, your frame should be fine. Just don't ride it with the loose fork, that could mess up your frame.

Chaves

p.s. be carful not to tighten the stem cap to much, you may bend the starfingle inside the fork which is a pain in the ass to replace unless you have a special tool.