Watts to volts
Watts alone do not give volts: you need a second quantity. With current, V = P ÷ I. With resistance, V = √(P × R). Sixty watts at five amps is twelve volts; sixty watts into ten ohms is 24.5 volts.
How to convert watts to volts
The reason a plain watts-to-volts converter cannot exist is that power is a product of two independent quantities. A 60 W load could be 12 V at 5 A, 230 V at 0.26 A, or anything else on the same curve. Where this conversion genuinely comes up is in reverse-engineering a component: you know a resistor dissipates a certain power and you know its value, and you want the voltage across it, which is the square-root form.
Questions
No. Power is volts times amps, so you need one of the other quantities.
V = P ÷ I if you know current, or V = √(P × R) if you know resistance.
Because P = V² ÷ R. Rearranging for V puts the power under a root.
For resistive loads with RMS values, yes. Reactive loads need impedance and a power factor.
Then the question has no single answer: any voltage is possible with the matching current.