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The three most common issues for hydraulic power assisted steering loss of assistance are snapped auxiliary belt, lack of fluid or failing pump.
As you don't mention any battery warning lights or other related issues to the failure of the auxiliary belt and the fluid level you report to be okay, the next thing to consider would be low output pressure from the powered steering pump or a blocked hose to the steering rack, the failing pump is more likely but hoses I have known to collapse internally before reducing the pressure supplied to the steering rack.
Are you handy with tools? This is usually a workshop issue to diagnose.
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