Your result
Possible
There are two questions here, they have different answers, and they are easy to run together.
Your parent's position. If a declaration of loss was published against them, they can very likely get it undone — by revocation of the act where the foreign naturalisation was effectively required of them, or by reacquisition otherwise, both decided by the Ministry of Justice, and both restoring the nationality they were born with. If no declaration was ever published, they never lost anything, which is the commonest outcome of all: loss was never automatic and only ever took effect from publication in the official gazette.
Your own position, which may be the better claim. Brazil has no generational limit on citizenship by descent, so if your parent was Brazilian when you were born, you have a claim of your own — and the date is what decides it. If the declaration against them was published after your birth, you were born to a Brazilian and your claim stands, whatever happened to them afterwards. If it was published before your birth, your claim has to wait on their nationality being restored first.
So the order of work depends on one date, and family memory is not reliable about it. Our citizenship-by-descent quiz covers your side of this.
Give us your parent's full name as it appears on Brazilian documents, their date of birth, and your own, and we will find the published date and tell you which of you applies first.