how come you've reached the ladder without ANY yellow keys???
not trying to spoil much here but, you'll need at least 2 yellow keys to progress
it's also very important that you don't use the ONLY spear in the level on any dumb enemy, you'll need it for the boss, throwing the spear on the floor right underneath him will cause it to collapse and send him into the trash compactor.
so yeah, if you used the spear already, you should restart everything