Gestures are just as important as phrases

If you forget the word for your gratitude, you can briefly look at the face of the person you are grateful for, and then tilt your head down forward. This should always work in any occasion.

Depending on your relationship with the person, the angle of the tilt would be different. If your relationship with the person is such that you are senior to the person or you are a customer at a restaurant or a hotel, then a slight and fast tilt would be enough. If your relationship is such that you are junior or you owe to the person for something else, you tilt your head down lower and hold it down for a few seconds.

Phrases you can use also depend on the relationship. If your relationship with the person is such that you are senior to the person or you are a customer at a restaurant or a hotel, then "arigatou" should be sufficient. If your relationship is such that you are junior or you owe to the person for something else, "arigatou gozaimasu" should be used. In the latter case, if you use "arigatou" alone, it would sound impolite to the person so you would be better off not using the phrase but tilt your head lower and for longer to show your deeper gratitude.