RK,
Hog & Dj are spot on in their advice.
You do have some access but if locating the cable, it may still be a bear to fish it back up.
I will bet that you didn't have much of a pigtail & that's why the wire fell thru.
If your wiring was abit short, I would pull out the existing wiring & cut it leaving a foot of length out the base opening. I would purchase the proper length of new wiring or an extension piece something a little extra in length. I would fish rigging wire or fish tape down as the top hole is small in diameter.
At the base opening, I would strip the ends of the new wiring back, twist them & solder together to loop the rigging wire or fish tape into, remember that top hole is small. Once connected, you can pull the new wire up or if in reverse pulling down. Once you have enough length of wire exiting the top hole, slip the grommet on & tie a knot in the wire so it won't slip thru again & pull from the bottom to get the knot against the grommet. then just cut the pigtails the correct length for mounting to the light.
CR