Failing that, you could uninstall Play Store (Home Screen > Menu > Settings > Applications > Manage Applications > All > Play store), which should revert it back to the Market app, and wait fo the next update to bring it back to the Play Store.
and/or
Google "google play apk" to find a source for the application file. Download it and run it (you may have to check the box that says "run applications from other sources" or whatever it's called) to install the application again.
I've had this problem with my Desire many times - normally I have this problem in combination with being unable to run Maps and the Navigation app, which also wipes my contacts and SMSs. Most annoying.
Rhyan