Multibit creates addresses using compressed private key, while Electrum uses uncompressed private keys. You can't decompress key - it will change public address, not allowing you to use your coins.
Check first letter of your exported private key from Electrum. If first letter is K or L, it's compressed. If it's 5, you have uncompressed private key.
Easiest way to change programs is to create new wallet in Electrum and send your coins to new address.
If you can't send it from Multibit, because it's broken, install bitcoin-qt (it supports compressed private keys).
To import private key with bitcoin-qt:
- Close bitcoin-qt
- Open console (press start > type "cmd")
- cd C:\Program Files (x86)\Bitcoin\daemon
- bitcoind.exe --deamon
- Open another console
- cd C:\Program Files (x86)\Bitcoin\daemon
- bitcoind importprivkey 5Jvhv9N8BHYAp9Uowvz28fvaqjeiHC3FcEb9p6X2SncnhnHUVyQ test3 false
- if there is no error, close both consoles and open bitcoin-qt and synchronize
There are probably more/better tutorials how to import private key to bitcoin-qt. Important thing is - you can't import compressed private key to program that doesn't support compressed private keys (like Electrum).