You may encounter this error message when you open MetaMask:
Message: Cannot read property 'balance' of undefined
We recommend you reset your account by following the instructions here. Before you do so, consult a member of the team by getting in touch with Support — resetting an account should be avoided where possible. Make sure you have your Secret Recovery Phrase backed up before you reset your account, as well as the seed phrases or private keys of any imported accounts.
One thing to keep in mind after resetting your account is that your transaction activity history may default to none: this is because MetaMask stores your transaction history locally. Remember that all transaction history is on the blockchain, and can be consulted on a block explorer. MetaMask being unable to retrieve your balance does not mean that your funds have been lost.