Here are some things you can try:
0) Reboot your computer
1) Rename your current Celestia directory to something else (e.g. MyCelestia), then download and install a pristine copy of Celestia from
http://sourceforge.net/projects/celestia/ and try running that.
If the new copy of Celestia crashes in the same way, then the problem is intrinsic to Celestia's interaction with your system, maybe a registry entry. If it doesn't crash, then the problem is due to come change you've made.
Assuming it's due to one of your changes:
2) Eliminate all of the addons: rename the "extras" folder somewhere outside your Celestia's directory tree, then try running Celestia.
If Celestia runs OK, then recreate the extras folder and copy your Addons back one at a time until Celestia fails.
3) One at a time, eliminate any modifications you've made to Celestia's configuration and/or data directories
e.g. move files that you've changed to some other directory, outside Celestia's directory tree and replace them by copying over the originals from the pristine version of Celestia installed in step 1 above.
I hope this helps a little.