If you are referring to an employer sponsored Retirement Plan, you can access your 401K balance, allocation, and contribution history by contacting your Human Resources Department. They will most likely direct you to an online portal for your Plan Sponsor. If you have not accessed this information before, you may need to register for this access. Upon receiving a log-in and Password, you should be able to track your 401K information as often as you like.
The best way to get this information is to contact the investment firm that is holding your money. If you have a statement or other document, there should be a telephone number on it for you to call.
You didn't mention what type of account you have. If the account is a 401(k) from an employer, you can contact that employer's HR (Human Resources) department. They should be able to put you in touch with the people managing the 401(k) plan for them. The same is true if this is a 401(a), SEP, SIMPLE IRA, or 403(b) plan. In all these cases the employer can point you in the right direction.
If the account is an IRA or ROTH IRA or some other type of account (e.g. a taxable investment account) then you will need to go to the company holding the money. If you don't have an account number the company will likely be able to find you via your social security number. You may also need to answer some additional security questions (it may feel frustrating, but it is to protect your information and privacy), but if you have an account, you should be able to get the information you need.