It was reported in the news recently that a large collection of email addresses and passwords used to hijack accounts on other services was discovered being distributed on a popular hacking forum. The data contained almost 2.7 billion records including 773 million unique email addresses alongside with passwords for these emails, used for other services. This is not new and we, as an IT service company, are constantly receiving questions of whether or not an email account has been stolen (largely as a result of email spoofing).
If you are concerned about your email account, here's a good tool to check if it was hacked and, if it was, where did the hack occur. The information is not exhaustive, but it gives an idea:
It is important to remember that passwords should be changed regularly and not duplicated on other services. The best option is to use a password generating tool, to avoid password replication.