If you get this error message when you open your MetaMask:
Message: Cannot read property 'balance' of undefined
If you've encountered this error, please reset your account by following the instructions here. One thing to keep in mind after resetting your account is that your transaction activity history may default to none. Of course, all transaction history is on the block chain, and can be consulted on a block explorer.
BE SURE TO SECURELY SAVE YOUR PASSWORD, Secret Recovery Phrase and PRIVATE KEYS (for imported accounts) BEFORE RESETTING.