Try deleting it and re installing it. That works sometimes. Do you have minecraft open? Is everything at the latest version? Has it worked in the past? Do you use mods?
Is it up to date with the most current version? Is your computer no longer compatible to run with the most current version of MCedit or are you using an earlier version of MCedit and your computer has been upgraded recently? Uninstalling and reinstalling it might help. If that doesn't work, I will put more things here that you can try.
Could you answer the questions I posted. It may help someone find the source of your problem. Make sure you are running the correct version. If you have a 32 bit computer then you need the x86 version while the x64 version is for 64bit computers. Also, have you deleted and re installed it?
Try deleting it and re installing it. That works sometimes. Tried 3 times Do you have minecraft open? No Is everything at the latest version? Yes Has it worked in the past? No Do you use mods? No
I have nothing. The only thing I can think of is that you have the wrong version but I find that very unlikely.