How can I fix a gap under my laminate flooring?

I just laminate flooring put in the dining room. As the floor wasnt 100% at level the fitter left a small gap in the underlay at the point on the flooring where the floor starts to slope downwards. Now above the gap is where there happens to be one of the joints for the flooring. Ended up stepping on this point and felt my foot digging in slightly followed by a slight cracking sound. Just took my foot of in time but now the planks are off level. This point is now very fragile. ANY WAY I COULD FIX THIS WITHOUT TAKING THE ENTIRE FLOOR BOARDING APART? ANY FILLER I COULD USE THE FILL THE GAP UNDER THE FLOORING?