Play Store gone from my Cyanogenmod 7 Galaxy Ace. How can I get it back?
Format the system Partition and reflash Cyanogenmod and Gapps in recovery. Unfortunately you will have to reinstall all apps (even ones on the SD card).

Note: Sometimes, bizarrely, it can wipe part of a SD card. I lost photos and videos and also appdata (which was ext4) but all my downloads and recovery zips remained. Not sure if that it a problem with the OS, my phone or the recovery system I use. Just thought I would mention it as a warning, just in case.