First and foremost, this is my first answer on a stack exchange website so please bear with me if its not exactly what your looking for.
In answering this question i am going to assume 3 things:
- You still have the ROM for your OS (or a Nandroid backup that will suffice)
- A custom recovery (such as Clockwork Mod or Amon RA) installed on the
- You have a RELIABLE method of re-installing the play store (such as a ROM
downloaded from a reliable source). I was able to find a good ROM by
googling "CM7 no app store" but im not able to find it at this time. If
someone could post a good link please do so.
I had a problem with my device not allowing me to install the play store at all. The only way I was able to fix it was with a complete restore of the system. So if you are willing to start from scratch (I know...) then I'm pretty sure this will fix the problem.
Ensure your battery is full before continuing
Some places say to have 40% others say 70%... I prefer to air on the side of caution and go with a FULL battery. Also, create a backup of the device before you do anything, if it doesn't work, you will want to be able to go back to before I broke your device ;)
Begin by backing up anything you feel needs to be saved, preferably to an external storage such as a flash drive or PC hard drive.
Wipe the device and the SD card (the same kind of wipe used before installing a new ROM)
Load the rooted android ROM (or the Nandroid backup) as well as the ROM or APK for the play store on to the SD card.
Boot the custom recovery (the method varies from device to device) and flash the custom ROM (or restore the Nandroid backup)
Once the ROM (or backup) is finished loading, power the device fully off, remove the battery and the SD card. and wait 5 minute. This is not standard but again I like to air on the side of caution, since in my experience with desktops this is a good practice.
Reassemble the device (this part IS required lol) and turn the device on. wait for it to load. Make as little change to the device as possible in getting to the desktop. Activation is not REQUIRED and you can activate the device at any time so i prefer to wait until all the gears are spinning to do things like that. To skip the activation procedure you just hit the back button. Dont change any settings, features, or even so much as open a menu.
Turn the device fully off. If your using a ROM for the play store then the next step is to go into the recovery mode and flash the ROM the same as your would a custom OS ROM only you DO NOT wipe anything. Just flash it on top of the current OS.
If your using an APK file then you would install it the same as any other 3rd party APP. You need to go into settings > applications and allow the installation of 3rd party applications. Then reboot the device.
If everything is working I recommend that you make a nandroid backup of the unchanged OS with the working play store. This would save ALOT of trouble in fixing any issues that come up in the future. I personally back up my device every week so i dont have too much fixing to do if something goes wrong...
If the play store still doesnt work, you probably have a bad apk or rom for it, and I would recommend that you attempt to find a different one that does work.
Again I appologize if this does not help, I personally am not very experienced with Androids and this is what I would do to try and correct the problem.