You need to find the Smokepowder Reserve that Philomeen left behind to clear the cave-in, but where exactly is it? Below is the location of the Smokepowder Reserve in Grymforge to complete the Find Explosives quest.
Philomeen is on the northern part of the map. From the area above, go north and down the staircase. Go east on the area littered with cages. Keep going down the hallway until you reach a double door with two guards.
Go through the door and find the button in the room. Go inside the opening and go up the staircase. Jump over to the other side to reach another set of Iron Double doors. You will reach a room and Philomeen will be in the middle of it. Talk to Philomeen until you get a vial of the Runepowder.
Clearing the Cave-in
Go back to the guards to find the Cave-in. Interact with the cave-in to activate a cutscene. You can continue or leave after the dialogue. Afterward, use your Runepowder vial and throw it on the cave-in to clear the rubble. Step back to avoid getting hit by the rubble.