The symptoms include getting locked out of your email address, your sent items containing strange emails you never sent, and your friends, family, and coworkers reporting they received SPAM from your account.
The first things you should do is change your password on all accounts, change your security verification information, and send a group email to everyone informing them of what’s happening. If you’re locked out, immediately report this to your email provider as well. There are a variety of things you can do to both prevent and deal with such occurrences.
In the future, ensure you are consistently changing your password, ensure it is a mix of characters and numbers, and keep a separate email address for any web sign-ups. Also, make sure your computer, your browser, and your antivirus/antimalware programs are not only up-to-date, but automatically being updated every day or week.