Generally, your airline, travel insurance or sometimes your credit card company will reimburse you for reasonable expenses like toiletries, clothing or other essentials if your bags are delayed for more than just a couple of hours. However, you must have receipts and have filed a claim as soon as you realize your bags are missing.
If your bags are ultimately determined to be permanently lost there is a set amount that is allowed by airlines, travel insurance and credit card or homeowner's insurance policies, but you may only be reimbursed for part of the cost.
To file a claim on lost luggage, go to the baggage counter at the airline as soon as you realize your bags are missing. They'll give you a form to fill out and make sure to be as detailed as possible with contact information, contents and other requested information.
If you do incur expenses, you might want to file a claim with your travel or homeowners insurance, or credit card company as well. They may be able to provide additional reimbursement that you were not able to receive from the airline.
In case this does happen to you it's important to have travel insurance. Another good idea is to pack medicines, extra clothes and other essentials you may need for an overnight of travel in your carry on bag.
Make sure you have tags on all your luggage and double check that they are going to the proper destination. Also, don't lose your baggage claim ticket, it can be very helpful in trying to locate your bags!