Right back to square 1

It can be the most difficult thing to do, but when a key aspect of the buying decision changes you have to go back to step one of your sales process. 

For example:

You find out that you are not talking to all the right people

You find out that the needs or priorities of the buying organization have changed

You find out that the budgetary situation is not as you anticipated

The customer tells you that the solution you have been pushing does not address their needs

You become aware of new competing choices being considered by customer.

So, what is square one?  Well listening to understand what the prospects needs are (implicit and explicit) and their business impact.  You have to revisit the buying process, the decision makers and influencers, the timeline, etc.   Go through all that again?  Yes.