Try rebooting the phone if you haven't done so yet.
Also, if you have Root Explorer... see if the Calender.apk is there... under System > Apps.
If you have a backup of all your apps with Titanium, you can check there also.
If its missing or was accidently deleted, see if you can find someone to upload the Calender.apk to a file sharing site OR you may need to flash the ROM again... make sure you're wiping all the caches and the data.
Thanks a lot guys. I went and just reflashed and it's working fine. I forgot to create a nandroid back upMake sure you're grabbing the calendar.apk for the newer Sense. The Thunderbolts running Sense 2.0.
I was trying to find you the apk but all I could find was calendars from the Incredible and Evo which uses the older version. I pulled the calendar off my phone and reinstalled its apk through Root Explorer to test it and it does work but as everyone else has said you're probably safer flashing the ROM all over again.
