hi i had a fiat punto aswell, that happened to me too,
it is electric,so no fluid required!!
it could be one of two things!!
1) the punto has that locking mechanism in place so if you turn the wheel too harshly it will lock itself. you just stop the car and start it again and it will be fine, now that should really only happen on occasion if you are really turning the wheel!! (its a stupid thing in the first place!!)
or it could be (2)
the steering rack or column is gone, this is what happened to me, in this case the locking of the wheel happens a LOT!!. in my case and i believe is quite common in puntos, when you turn left! wahtever thats all about!!
i would defo bring it into a garage because i had to have the whole steering column replaced!! if this is whats wrong try get a second hand one, which is prob gonna cost you like 500 quid or something. new it will cost you about 1000-1500!! and if your car is a 2000 model like mine was your car is prob only worth that much in the first place.
so weigh up your options. sounds like that whats wrong to me though, so bring it in a see what they say,..puntos.. a complete pain!!!