You can always access your webmail from any internet-connected browser. To access your webmail, simply enter webmail.yourdomain.com or yourdomain.com/webmail into the browser. Just replace 'yourdomain.com' with your own domain name. Then log in with your email and password.
If you have access to your control panel, you can also access your webmail from there. Log in to the control panel, click on mail accounts in the menu, and then click the 'Check Email' button.