How do I find the hypotenuse of a triangle when given 90 degrees and one side length?

You need one more fact either an angle or a side in order to be able to answer. I take it you have neither. But there must be something more you can tell us from the question. List the question exactly as it is asked with a diagram if necessary. I'll follow and use either comments or I'll edit.

The direct answer to your question is that you can't.