check tyre pressure's and steering geometry, failing that it could be down to a worn or dry steering rack,Answer
I have had this exact problem last week, If you find that your tyre pressures are correct or after adjusting them to the correct pressure your steering is still stiff, I suggest you get it to a garge as soon as possible, The problem may be with the steerign rack, if this is the case, then it will be the universal joint (UJ) that has collapsed, in which case they will need to replace the Uuiversal joint and the steering rack for you. This is the problem I had. It costs about �100 then you add V.A.T at 17.5% so look at �117.50 to have this replaced, the price may vary slightly from garage to garage but its in that price band, and shouldn't cost much more than that.
An easy way of detemining whether it could be the UJ collapsed is, what happens, when you say your steerig ggoes stiff. if you find that you are driving along and the wheel is so stiff it feels as though it may lock on you or its very difficult to turn then it is that the universal joint has collasped. If this is the case do not drive around in the car as its unsafe and you could have an accident, book it into the garage and get it sorted.Also check drive belt has not snapped or is slipping as this drives Power Steering pump.
if its a power steering model its more than likely a power steering fault, underinflated tires are really noticable unless they're REALLY flat