are you using home, or panda home or any other laucher? Try holding down the home button and it should bring up which launcher you want to use
That probably won't do it - holding down home lists the last six apps that you are running.
What has happened is that the default home application has switched from Sense to Home. To fix this, there is a free app in the market called "home switcher", or, if you want to do it without the app:
Settings->Applications->Manage Applications.
Wait for the list to refresh.
Find the app "Home". Tap it. Scroll down to find the button "clear defaults" and tap it. Press the home button - it should ask if you want to open HTC Sense or Home. Tap the checkmark to make the selection the default, and choose HTC Sense. You should be all set.