The Highlander star, 69, fell ill on Saturday while meeting fans, prompting staff to call emergency responders and escort him from the venue, a witness informed TMZ.
Convention organisers later confirmed Lambert had left due to a "personal issue" and his scheduled photo op was cancelled.
Witnesses said he appeared shaky before being placed in an ambulance and taken to a nearby hospital for evaluation.
Lambert's representatives have since said he is "perfectly OK" and recuperating in his hotel room.Â
According to his team, he had barely slept the night before and hadn't eaten enough during the day, causing his blood sugar to drop and triggering the collapse.
The French actor has faced health setbacks before. Last year, Lambert suffered a serious fall that forced him to withdraw from an appearance at the SciFiWorld festival in Sweden.
At the time, his team explained that filming commitments had to take priority and he was unable to attend while recovering from the injury.
Lambert, known for roles in Highlander, Highlander II: The Quickening, Mortal Kombat and Fortress, regularly meets fans around the world.
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