how can i fix my yamaha neos 100?

hi, i have a yamaha neos 100, i took off the wheels to change the tires, when putting wheels on, had trouble getting back wheel on but got there in the end, and as for front, got it on but when i tighten the nuts the wheels does not move if i loosen the nut the wheel goes round but its no good as the nut is loose,, please can anyone help me, many thanks, Steve,,