There’s not a black and white answer to your question.
These kinds of issues are best addressed in a contract, which I’m assuming you didn’t have. If you designed it through a crowdsourcing site, whatever contract they might have with you should determine the answer to your question.
If no contract exists or nothing in the contract mentions anything about using the work for your self-promotion, it gets a little fuzzy. As the creator, you arguably retain some rights to use the work, but that depends on the country you live in and what the laws might be there.
In the US, unless the contract says otherwise, you still own the creation and have just sold usage rights unless you’re an employee of that company. However, when I create a logo for someone, as far as I’m concerned, it’s 100% theirs. I specify that all copyrights to the work transfer to the client upon payment in full in my contract.
I don’t know what your situation might be, but if your client prefers that you not use their logo for self-promotion, I’d respect their wishes. Don’t assume you can just use their logo — it’s best to ask your client and get their answer in writing or an email.