In Pokémon HeartGold and SoulSilver, each slide puzzle that is
completed unlocks a specific subset of the 26 alphabetic forms of
Unown. To unlock all 26 alphabetic forms, all four slide puzzles must
be activated. The slide puzzles can be completed in any order; the
first completed slide puzzle will cause a man to give the player the
Unown Report. Once the first seven pages of the report are complete
and the 26 alphabetic Unown forms have been caught, the entrance room
of the ruins will have a gold plate that reads "Our friends look up
and heartily congratulate us". Reading the plate causes one of the
researchers to speak to the player, after which the ! and ? forms can
be encountered in the entrance room.
Check the entrance again. It should be there after you complete the 4 puzzles and catch all 26 Unown
Source: Bulbapedia