Some Sigma lenses feature a "zoom lock" to protect the lens when not in use, so just make absolutely sure that this hasn't accidentally been left engaged nor that the switch is being obstructed from unlocking somehow, such as by dirt/debris. It's also worth giving the lens a few sharp taps down onto a table in case it's just a bit of sand or other debris.
Otherwise, it's not a DIY fix due to the complexity of modern autofocus zoom lenses. If you try to force the lens or attempt an incompetent repair you'll ruin a £300 lens.
The best option is to send it to
Sigma for a
repair estimate, if you're in the UK then there's a
fixed fee of £93.99 (tariff no.6). The link I've provided is for the UK but if you're elsewhere different fees and procedures probably apply. If it helps, my experiences of getting similar repairs done in the north America is that they're usually a bit cheaper and in Aus/NZ they're a bit more expensive.
If your lens has failed under warranty then the repair is free if the lens has not been misused, there is no evidence of any unauthorised repair attempt, and you have proof of purchase.
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