You should immediately:
* Pull over safely as soon as possible. Driving with a serious transmission problem can cause further damage.
* Check your transmission fluid level. If it's low, adding fluid might temporarily alleviate the issue (but doesn't fix the underlying problem). However, do not overfill.
* Consult your owner's manual. It might provide more specific information about this warning light.
* Have the car towed to a qualified mechanic. They can properly diagnose the problem and provide the necessary repairs.
Ignoring this warning light could lead to significant and costly repairs down the road. It's best to address it promptly.