I AM NOT A LAWER - Just a freelancer :)
Depends on the sum of money which is being withheld.
In the instance of a $2500 USD project, it may not be worth the headache to chase the client. In that case, take it as a loss.
If it's a $25000 USD project, you may need to consult your lawyer.
1. Ensure your client signed your contract
Contract should define the "rules of the game" both parties agree to follow.
2. Ensure you have client contact information prior to starting project.
This can be defined in the contract as the "point person" and at least one back-up person in case the point person is on vacation, unreachable, etc.
3. Properly define who owns the work within the contract.
A lot of contracts assume work-for-hire which can mean the client owns all the work at any given point throughout a project.
If you do not have this information defined in your contract add it! Ensure that rights to the work are yours until the client pays in full, and at that point, the rights are transferred to the client. Some contracts go as far as stating the creator always has ownership of work.
4. Reach out to back-up contact person
If the company is large enough there may be another team member playing support on the project. This could be a colleague or a superior at the clients company. If necessary, CC this person on emails to garner attention and a response.
5. In some cases, if the remaining sum of money is large enough, it may warrant reaching out to your lawyer
If there is a large amount of money remaining to be paid some instances may warrant legal attention. It could be as small as a letter from your lawyer or legal action.
CONSULT A LAWYER BEFORE ACTING.
This is the nuclear option. Use it carefully, if at all.