LastPass noticed a possible security issue with one of their databases and is now requiring some users to change their master password. If you haven't seen the notice yet, this may explain why you might be having issues with the service.
LastPass wrote on their blog yesterday that because they can't account for the anomaly they detected in one of the databases, they're going to be paranoid about it, even though if you have a strong, non-dictionary-based password you shouldn't be impacted.
I didn't receive a warning or message to change my master password, but LastPass wrote in the blog comments that right now they're only forcing master password changes if you try to use the service from an IP address you haven't used recently. Also, trying to access LastPass from a mobile device can cause you to be locked out, according to some users in the comments, so if you try to change your master password, don't do it from a mobile device.