Need a bit more info than that. What rom did you have before, what did you flash, how did you do it, does it go into recovery and if so, does it show any error message, how do you know it's the /data partition that's not getting mounted.
If its really the /data partition, you can try to go into recovery, get adb connected and mount that partition manually. In my S3 it's on /dev/block/mmcblk0p12
I do believe odin does restore /data, if so, that's not your problem.
Doesn't do /efs though. I had a similliar problem once with the /esf partition. Had to manually format and copy it's files back. Only then my phone booted properly again.