Nothing stopping you from a career change as long as you have realistic expectations. For example, if your sales career was paying you $100K, don't expect to start in a new field at the same rate. Look at what others in the field are getting for starting salary and base any job prospects on that.
As for learning ASP.Net, this really depends on the individual. Certainly it is easier if you know another development language to start, but it isn't required. You'll want to understand web development in general too since any interactive pages could require Javascript, VBScript, back end databases, integration into various applications, etc... I know it may sound a bit challenging, but it really just takes an interest and opportunity. Can't really say how long it would take since each person learns at a different rate. Your past skills also factor in so you may take longer to get up to speed. What you may want to do is try to combine your skills and look at sales related web development too.
