I think I braked too quick on a slope and the brake cable came out of the brake handle so my front brake doesn't work.
What do I do to fix it?
The brake outer cable and ferrule most probably popped out of the barrel adjuster on the brake lever.
This can happen when you panic brake if your brakes were a little out of adjustment -- basically you pulled in a ton of inner cable while you were saying "ohhhh mmmyyyyyyy" and then when you let go of your death grip, the inner cable went back out and as your brake liners were a bit sticky, brought the outer with it.
You should be able to pop the outer cable back into the barrel adjuster with just some arm strength. Backing the barrel adjuster all the way back into the brake lever (or on the brakes itself) will help. Many brakes also have a detensioner mechanism which is designed to make it easy to detension the brakes (so you can take the wheel off).
Once you get it back on, to prevent it from happening again:
Adjust your brakes so that you don't have to bottom out your levers in order to get maximum power <== most important
If you have the dough (US$5-15), get new brake cables (inners, outers) - this will prevent the stiction that makes it easier for them to pop out.
If you do #1 properly, #2 should be unnecessary.
This can happen if there is too much friction between the cable and the housing, usually from a corroded brake cable or from a bent piece of housing. Replacing the cable and housing would be the best solution but in the mean time it can also help to lubricate the cable or manually unkink the housing.