I had a midas blenny do the same thing in my 14 gallon Biocube a couple of weeks ago. The ONLY thing that worked for me, after trying to flood the back compartments and trying to get him to swim back over, using a brine shrimp net, etc.. was to siphon him out. Worked like a charm! I went out and bought some of that hard plastic cross-stitch mat and made a cover for those back compartments so that it wouldn't happen again!
Try that and see if it works!