Lord Rama constructed the Setu from Indian mainland to then Srilanka with help of the vanara army. After the death Ravana, what happened to the stone bridge?
According to Kurma Purana, the bridge will exist as long as the earth is there.
In the 21th chapter of the Purva Bhaga of Kurma Purana, Ramayana is narrated in brief.
With the mere look at the Rameshwara Linga, all the sins of a person are washed out. There is not a slightest doubt about it. Till the mountains are there on earth and the earth remains, the bridge will continue here. I shall stay here in invisible form. The baths taken here and the charities performed at this place shall remain without decaying. The Sraddhas performed here also won't decay and will be everlasting. (50-51)
Present day Sri Lanka was not Ravana's Lanka.
Further, according to the Ocean God, who assured Sri Rama, that the bridge being constructed would allow the passage to Him and the vanara-s assisting Him.
विधास्ये येन गन्तासि विषहिष्ये ह्यहम् तथा | न ग्राहा विधमिष्यन्ति यावत्सेना तरिष्यति || २-२२-२८ हरीणाम् तरणे राम करिष्यामि यथास्थलम् |
"O, Rama! I shall make it possible to see that you are able to cross over. I will arrange a place for the monkeys to cross me and bear with it. As far as the army crosses me, the crocodiles will not be aggressive to them."
अयम् सौम्य नलो नाम तनुजो विश्व कर्मणः | पित्रा दत्त वरः श्रीमान् प्रतिमो विश्व कर्मणः || २-२२-४४
"O, excellent man! This one, named Nala, a glorious person, is the son of Vishvakarma; who was given a boon by his father and is equal to Visvakarma."
एष सेतुम् महाउत्साहः करोतु मयि वानरः | तम् अहम् धारयिष्यामि तथा हि एष यथा पिता || २-२२-४५
"Let this greatly energetic monkey build a bridge across me. I can hold that bridge. He is just the same as his father."
So the bridge got constructed by Sri Rama would be in place, as long as He and his army passes over it, ie., it will remain intact for their to and fro journeys. There after it will collapse.
Sri Rama performed the return journey by Pushpaka Vimana and hence the bridge would have been collapsed immediately thereafter.