From
Troubleshooting - CyanogenMod Wiki
This typically happens during a ROM upgrade or when moving sdcards with apps2sd or similar. The cause is the application file (.apk) actually goes missing or gets deleted, but the market still thinks it's installed. The easiest solution is to manually remove the market data and let it re-create it from the .apks that are present.
- Start by making sure the market isn't running (task manager of your choice, or settings->applications->manage applications, Market, 'Force Stop' button).
- Then open a terminal and type
su
rm -r /data/data/com.android.vending/*
Finally, restart the market (it may take a minute and possibly throw an "Application Not Responding", just press wait)