Main drain problems

Hello. I have a huge problem with my in ground vinyl pool. I went out this past weekend and noticed the pool cover was lower than usual. Upon further inspection I pulled back the cover only to find that almost all of the water was gone from my 26,000 gallon pool. Upon closure I drained the pool to about 1 " below the skimmers, blew out all the lines, put in anti-freeze and plugged the skimmers like I have since 1999. Now I suspect I have a crack in my main drain line somewhere. I am assuming that the vinyl pool has a steel shell with metal lateral bracing. I don't know how the main drain is installed, but I think it is set into the steel shell on the bottom. If the line is cracked mid-way under the pool how do I go about replacing the line? The original lines were flexible PVC. I know this because I had a leak problem years ago in the skimmer lines, which I dug up and replaced all skimmer and supply lines with schedule 40 PVC. The only line I didn't replace was the main drain line. What is the best way to proceed? I know the location of the main drain line where is exits the pool bottom and it is under the skimmer closest to the filter. Any assistance would be appreciated.