It is fairly common for email accounts - typical email account with webmail services such as Hotmail, Yahoo!, Email, Mail.com - to be compromised and hacked by malicious individuals. The purpose is usually to hijack the account and send email spam and phishing email from it.
The root of the problem is often an insecure home computer: individuals with malicious intentions can install keylogging software to record key strokes and therefore record your username's and passwords. It is possible for hackers to gain access to your PC through remote control programs and gain your username's and passwords in countless ways.
So, first of all, install and run some up-to-date anti-virus and anti-malware software to make sure the above is not the root of the problem. Also, install a password onto your PC so that others in your household cannot access your home computers user account.
Once that is done, the next step is to reset the password on your email account. Make the password strong: over eight characters; numbers and letters; and in lower and upper case characters. That should be enough to re-secure your account; however, you should continue to run the anti-virus software on a regular basis, update Windows on a regular basis, and, change your email account password - you have guessed it - on a regular basis.