As long as you haven’t been hiding under a rock for the past week or so, you’ve most likely heard of a fairly catastrophic security issue called “Heartbleed.”
Rather than boring you with the details of what Heartbleed is, I found a great video that wonderfully describes it and its ramifications from a high level:
And, rather me tell you what to do, I’ll let Brian Krebs, one of the foremost security journalists tell you: Krebs on Security: Hearbleed Bug: What can you do?
Finally, if all you want to do is check if a particular website is vulnerable, I suggest using the tool from possible.lv: http://possible.lv/tools/hb/. This tool will also be able to tell you if a particular websites certificate has been changed since the vulnerability was discovered.