There is likely a login process that a participant would have to go through. Contact your HR department to obtain the web address and then follow the login process. If your do not know your login or password, many websites will have an option for a new user to setup an account which will tie to your information.
If you have a paper statement from the past or you know the servicing company for your 401k, you could try going to the website of the servicing company that is on that statement (ex: Fidelity). Once you arrive at that website you may be able to follow the process for setting up a new user that John mentioned above. If not then the best thing as suggested is to contact HR. They should be able to give you instructions on how to access it if online access is available.
If you cant find a statement with the website, just call HR and get the phone number of the plan administrator. The plan administrator will walk you through the very simple process of registering to the online portal. Also, if you can't find a statement, take this opportunity to be sure your address is correct