Unfortunately this is a very common problem with these head units. If you buy another one from someone else, there's no guarantee that it will stay working for long. Similarly, Lexus could fix it for you, but it would be at a high price and will probably just go wrong again at some point.
Your best bet is to buy a new aftermarket head unit, at least it will have a guarantee and should be reliable (if you buy a decent one). See the Audio/Video forum for details on how to replace your head unit.