If you are unable to answer a specific question because you don't know or don't have access to the appropriate information, you must indicate the reasons. You may refer to a previous response when responding to an interrogatory providing the previous response sufficiently answers the later interrogatory.
If you do not respond, the other side may ask the judge to order that all the facts are true or documents are genuine. This can often cause you to lose your case.
Courts enforce their orders by imposing sanctions on a party who fails to comply. Sanctions can be monetary, such as requiring one party to pay the other parties attorneys' fees and/or imposing a monetary fine, or they can hamper a party's ability to put on their case.
If you do not answer the questions by the deadline, which is usually about a month, the other side could ask the judge to order you to respond to the interrogatories. If you miss the second deadline, the judge could impose a fine against you or strike your pleadings.
If you have received discovery requests (which would probably come in the mail), you have thirty days to mail your written responses back to the other side. Missing that thirty-day deadline can be serious. It could even result in you losing the case.