I don't doubt when you say you draw a blank when asked what you want, but you may be falling into the same trap that many do: They limit themselves to what they think is 'realistic' which is probably what your parents (or whoever) actually meant when they gave you that instruction.
I would recommend that you first get rid of that limitation if it is there in your head. Impose NO limitation on what you think is possible before asking yourself what you want. If after doing that for a reasonable time you still come up empty then you're screwed, but I doubt that will happen.
If you do come up with something that appeals, start moving in that direction. No matter how small the steps, as long as the idea appeals to you, keep moving toward it.
But beware of a secondary trap. I did want all the usual things - wife, kids, house, car, toys of every description, money, freedom from any onerous task, free time, etc. Given my start in life those things seemed unlikely but the formula above worked. I got all that and more but ultimately it was not satisfying. After much pondering I came to know the only thing required for happiness. It is simply this:
All we need to be happy is something to look forward to.
If you really can't come up with anything, **** it, try drugs.