Most cases will see snapin locking tabs that hold the front inplace. You simply have to reverse the process to fit that back on. Removing the 5.25" drive bay covers will provide a better look as well as removing the side covers for a close look inside to see anything is fastened on the inside.
there are 8 tabs holding the top panel on,the first 2 are near the front of the case on either side of the 5.25 drive bays, next 2 are near where the motherboard tray ends, the next 2 are by the hole on the top of the motherboard tray(1 is on the back side of the mb tray), the last 2 are in the back of the case above everything else( they are both on the "guts" side of the mb tray)
If the front goes right on without the need to remove the top it will go a lot faster and easier being less likely to see anything break off. Since that was removed without lifting the top off you would just reverse the process to get it back on without any extra work.
A good close up look inside at the front would be even more of a help for jester070993 in case there is anything else besides the bottom two tabs he found already. The rest is simply fitting it into place and then using just enough pressure to see the tabs go in and lock.
With the 8 tabs you're going to push up those rather easily until each one pops out in order to see the top portion removed. Those lock in the same as the two you saw on the front plate but see more of them there.