Google Voice missing from app drawer after upgrading to ICS
If you to Settings --> Apps --> All, is it in the list of applications?  If so, click on it, click Uninstall, and then try to reinstall it from the market.

It's also possible that the market is incorrectly showing it as installed.  If this is the case, try going to Settings --> Apps --> All --> Market, click Force Stop, then Clear data.  The open the market again and see if Google Voice is still listed as installed, and if not, install it.