It broke on its own.
'Si è rotto' — si-eh-ROT-to. Flow the three words together.
Use when an item broke without any impact or misuse — a strap snapping, a handle breaking, a zip coming off. 'Da solo' (by itself) implies it was not your fault.
'Si è rotto' is the reflexive passato prossimo of 'rompersi' (to break, intransitive). The reflexive 'si' + auxiliary 'è' is used for involuntary breakages. 'Da solo' means 'by itself'. This construction is very natural in Italian and signals the item had a structural weakness.
Non l'ho urtato — si è rotto da solo.
I didn't knock it — it broke by itself.
Explicitly denies misuse.
La maniglia si è staccata.
The handle came off.
Specific component.
È fragile per un prodotto del genere.
It is fragile for a product like this.
Implies poor manufacturing.
The phrase 'si è rotto da solo' is widely understood and used in Italy. If a shop assistant suggests you caused the damage, calmly ask them to explain what normal use could have caused it — this often ends the argument.