The Verdict: Need More Info
This turns on whether anything about you was ever published in the Royal Gazette, and most often nothing was. Ask a Thai embassy, or your family's district office, what your record shows.
Since 1965 not one involuntary loss for naturalising abroad has been published, and many thousands of Thais hold a second citizenship and a Thai passport. Nationality decisions take effect only on publication.
Nationality is genuinely lost by:
A renunciation you signed that was then published.
A published revocation, which is rare.
Taking an alien identification document in Thailand, if your father was not Thai (section 21).
If you still have an entry on a house registration (tabien baan), that is a strong sign nothing was removed. Do not pay anyone to recover nationality until you know you lost it.
If you did lose it, a woman who renounced to take a husband's nationality can resume under section 23 once that marriage ends. Someone who lost it as a child has until 22 under section 24. Anyone else can use section 11, which waives the residence and language tests.