Is the mirror in the up position and is there a gap visible in the film gate?
IF there is, you can try moving the curtain away from the gap(open the film aperture fully) and let it snap shut.
BTW someone up there will rant about this not being proper. If it fixes it and costs nothing it's proper.
Use a fingernail(toothpick works too) and only apply pressure to the edge of the curtain. Nothing forward or backward.